diff --git a/docs/how_tos/feedback.rst b/docs/how_tos/feedback.rst
new file mode 100644
index 000000000..27368f4fa
--- /dev/null
+++ b/docs/how_tos/feedback.rst
@@ -0,0 +1,409 @@
+Feedback HOWTO
+==============
+
+Status: Draft
+
+
+Introduction
+------------
+
+frontend-app-payment has a `feedback module`_ that given proper configuration
+will display messages to the user that can be crafted by any API
+frontend-app-payment contacts.
+
+This HOWTO explains how the API should act to use this feedback module within
+the `payment module`_ and overviews where and how frontend-app-payment puts the
+message from the API's response.
+
+.. _feedback module: https://github.com/openedx/frontend-app-payment/tree/master/src/feedback
+.. _payment module: https://github.com/openedx/frontend-app-payment/tree/master/src/payment
+
+.. contents:: Table of Contents
+
+
+What should the API do?
+-----------------------
+
+Return a JSON response with in one of the following formats.
+
+The feedback module will ignore dictionary keys not listed in the formats
+below. Thus, it is safe to include additional keys in the API's response.
+
+Send the JSON response with a HTTP status code of a `client error (4xx)`_ or
+`server error (5xx)`_ for all formats except `List of messages`_.
+
+The implementation of these formats are in
+`payment/data/handleRequestError.js`_, which transforms these formats into the
+format required by `feedback/data/sagas.js`_.
+
+.. _client error (4xx): https://developer.mozilla.org/en-US/docs/Web/HTTP/Status#client_error_responses
+.. _server error (5xx): https://developer.mozilla.org/en-US/docs/Web/HTTP/Status#server_error_responses
+.. _feedback/data/sagas.js: https://github.com/openedx/frontend-app-payment/blob/master/src/feedback/data/sagas.js
+
+.. _List of messages:
+
+Format: List of messages
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: json
+
+    {
+        "messages": [
+            {
+                "data": "",
+                "code": "",
+                "message_type": "",
+                "user_message": ""
+            },
+            {
+                "data": "",
+                "code": "",
+                "message_type": "",
+                "user_message": ""
+            },
+            <...>
+        ]
+    }
+
+Send this format with a HTTP code for a `successful response (2xx)`_.
+
+.. _successful response (2xx): https://developer.mozilla.org/en-US/docs/Web/HTTP/Status#successful_responses
+
+.. _List of errors:
+
+Format: List of errors
+~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: json
+
+    {
+        "errors": [
+            {
+                "data": "",
+                "error_code": "",
+                "message_type": "",
+                "user_message": ""
+            },
+            {
+                "data": "",
+                "error_code": "",
+                "message_type": "",
+                "user_message": ""
+            },
+            <...>
+        ]
+    }
+
+This format is identical to the `List of errors`_, but uses ``error_code``
+instead of ``code``.
+
+.. _Dictionary of field errors:
+
+Format: Dictionary of field errors
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: json
+
+    {
+        "field_errors": {
+            "<field_name_1>": {
+                "error_code": "",
+                "user_message": ""
+            },
+            "<field_name_2>": {
+                "error_code": "",
+                "user_message": ""
+            },
+            <...>
+        }
+    }
+
+``<field_name_1>`` and ``<field_name_2>`` are placeholders for the `name`_ of
+the `Redux Form Field`_ the error message should be placed next to.
+
+The handling of ``field_errors`` is done by `Redux Form's stopSubmit`_ in
+`handleReduxFormValidationErrors()`_. ``handleReduxFormValidationErrors()`` transforms
+``field_errors`` into the format required by `Redux Form's SubmissionError`_.
+
+.. _name: https://redux-form.com/8.3.0/docs/api/field.md/#-code-name-string-code-required-
+.. _Redux Form's stopSubmit: https://redux-form.com/8.3.0/docs/api/actioncreators.md/#-code-stopsubmit-form-string-errors-object-code-
+.. _Redux Form's SubmissionError: https://redux-form.com/8.3.0/docs/api/submissionerror.md/
+.. _handleReduxFormValidationErrors(): https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/sagas.js#L59-L64
+
+.. _Single error:
+
+Format: Single error
+~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: json
+
+    {
+        "error_code": "",
+        "user_message": ""
+    }
+
+.. note::
+
+    For reasons unknown, this format undergoes some `custom handling`_
+    exclusively for the ``handleSubmitPayment`` saga.
+
+.. _custom handling: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/sagas.js#L243-L244
+
+
+What values should the API use?
+-------------------------------
+
+All values are optional, but the API must specify at least ``code``,
+``error_code``, or ``user_message``.
+
+``user_message`` is a string with the message that the user should see.
+
+``code`` or ``error_code`` are strings that usually identify a pre-built
+`AlertMessage`_ component to show. It overrides ``user_message``.
+
+``data`` will be passed to the pre-built ``<AlertMessage>`` component in a prop
+called ``values``.
+
+``message_type`` is a string that changes the ``alertType`` of the
+`StatusAlert`_ shown. See `MESSAGE_TYPE to ALERT_TYPE mapping`_ for valid
+values of ``message_type`` and which ``alertType`` they will trigger.
+
+.. _StatusAlert: https://paragon-openedx.netlify.app/components/statusalert/
+.. _AlertMessage: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/feedback/AlertMessage.jsx
+.. _MESSAGE_TYPE to ALERT_TYPE mapping: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/feedback/AlertMessage.jsx#L7C1-L13
+
+
+Which format should the API use?
+--------------------------------
+
+The `List of messages`_ and `List of errors`_ will put the feedback in any
+`AlertList`_ on the page.
+
+The `Dictionary of field errors`_ will put feedback next to the `Redux
+Form Field`_ whose ``name`` matches a key in the dictionary of field errors.
+
+Sending the message in the `Single error`_ format will result in the same
+experience as sending the message in the `List of errors`_ format with a
+list of errors of length 1.
+
+.. _AlertList: https://github.com/openedx/frontend-app-payment/blob/master/src/feedback/AlertList.jsx
+.. _Redux Form Field: https://redux-form.com/8.3.0/docs/api/field.md/
+
+
+What error codes can I use?
+---------------------------
+
+Some error codes are connected to pre-built components. See:
+
+* `FormattedAlertList`_ for the `AlertList`_ component
+* `AlertCodeMessages`_ for the formatted and internationalized `AlertMessage`_
+  components.
+
+.. _FormattedAlertList: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/components/formatted-alert-list/FormattedAlertList.jsx#L34-L58
+.. _AlertCodeMessages: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/AlertCodeMessages.jsx
+
+
+How does the feedback module work?
+----------------------------------
+
+Via exceptions
+~~~~~~~~~~~~~~
+
+* `payment/data/service.js`_ catches most exceptions thrown by the Axios client
+  calling the APIs and runs `handleBasketApiError()`_
+
+* `handleBasketApiError()`_ calls `payment/data/handleRequestError.js`_, which
+  reformats and transforms the feedback module JSON and re-throws the error.
+
+* `payment/data/sagas.js`_ catches most errors thrown by
+  `payment/data/service.js`_ and calls `feedback/data/sagas.js`_
+  `handleErrors()`_.
+
+* `handleErrors()`_ uses the ``addMessage()`` action and the associated
+  `addMessage() reducer helper function`_ to store the feedback message in
+  Redux's store.
+
+* `feedback/AlertList.jsx`_ is connected to the Redux store and formats each
+  feedback message into the props of a new `feedback/AlertMessage.jsx`_.
+
+.. _addMessage() reducer helper function: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/feedback/data/reducers.js#L22-L34
+.. _feedback/AlertList.jsx: https://github.com/openedx/frontend-app-payment/blob/master/src/feedback/AlertList.jsx
+.. _feedback/AlertMessage.jsx: https://github.com/openedx/frontend-app-payment/blob/master/src/feedback/AlertMessage.jsx
+
+.. _Via messages:
+
+Via messages
+~~~~~~~~~~~~
+
+* `payment/data/sagas.js`_ passes selected API responses into
+  `handleMessages()`_.
+
+* `handleMessages()`_ works similarly to `handleErrors()`_. See above.
+
+.. _handleMessages(): https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/feedback/data/sagas.js#L39
+
+
+How to clear the list of feedback?
+----------------------------------
+
+In `payment/data/sagas.js`_, either:
+
+* Run `feedback/data/actions.js`_ ``clearMessages()``
+
+* Pass ``true`` for ``clearExistingMessages`` when calling `handleErrors()`_ or
+  `handleMessages()`_.
+
+
+.. _feedback/data/actions.js: https://github.com/openedx/frontend-app-payment/blob/master/src/feedback/data/actions.js
+
+
+How do I send an error message through a URL parameter?
+-------------------------------------------------------
+
+`handleMessages()`_ accepts a third parameter, which is meant to be
+``window.location.search``, the `query string`_ of the user's URL.
+
+``handleMessages()`` will put any parameter of the query string called
+``error_message`` into an error-type message and show it to the user.
+
+See the `URL parameter error message implementation`_.
+
+.. _query string: https://developer.mozilla.org/en-US/docs/Web/API/Location/search
+.. _URL parameter error message implementation: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/feedback/data/sagas.js#L49-L53
+
+
+How do I add a new error message in the payment module?
+-------------------------------------------------------
+
+* In `payment/data/service.js`_, add ``.catch(handleBasketApiError)`` to the
+  method chain of the Axios request (usually ``post()`` or ``get()``) to
+  reformat the error into the correct format, then rethrow.
+
+    * `handleBasketApiError()`_ is a thin wrapper around
+      `payment/data/handleRequestError.js`_.
+
+* In `payment/data/sagas.js`_, add a try-catch that catches the error
+  reformatted and rethrown by `handleBasketApiError()`_ and calls the
+  appropriate error handler:
+
+    * ``yield call(handleErrors, error, clearExistingMessages)`` for the
+      `List of errors`_ or `Single error`_ formats
+    * ``yield call(handleReduxFormValidationErrors, error,
+      clearExistingMessages)`` for the `Dictionary of field errors`_ format
+
+* In `payment/AlertCodeMessages.jsx`_, add a new component for your message.
+
+* In `components/formatted-alert-list/FormattedAlertList.jsx`_, add the new
+  component you created in ``<AlertCodeMessages>`` to ``messagesCode`` under
+  key named after the ``error_code`` you want the API to call to trigger this
+  error message.
+
+.. _payment/data/service.js: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/service.js
+.. _payment/data/handleRequestError.js: https://github.com/openedx/frontend-app-payment/blob/master/src/payment/data/handleRequestError.js
+.. _payment/data/sagas.js: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/sagas.js
+.. _payment/AlertCodeMessages.jsx: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/AlertCodeMessages.jsx
+.. _components/formatted-alert-list/FormattedAlertList.jsx: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/components/formatted-alert-list/FormattedAlertList.jsx
+
+
+What is the fallback error?
+---------------------------
+
+If a saga calls `handleErrors()`_ but the API response is not in one of the
+formats above or ``error_code`` is ``fallback-error``, ``handleErrors()`` will
+throw up the `FallbackErrorMessage`_.
+
+.. _FallbackErrorMessage: https://github.com/openedx/frontend-app-payment/blob/master/src/feedback/FallbackErrorMessage.jsx
+.. _handleErrors(): https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/feedback/data/sagas.js#L10C22-L10C22
+
+
+When are snake_case dictionary keys auto-converted to camelCase?
+----------------------------------------------------------------
+
+The payment module's `payment/data/service.js`_ (also dubbed the
+`PaymentApiService`_ in payment/data/sagas.js) funnels many but not all API
+responses through `transformResults`_, which recursively converts snake_case
+dictionary keys of the response into camelCase.
+
+This was done because the convention for Python backends generating the API
+responses is to use snake_case, while the convention for JavaScript frontends
+processing the API responses is to use camelCase.
+
+See `payment/data/service.js`_ for if and when `transformResults`_ is used.
+
+`payment/data/handleRequestError.js`_ also converts the top-level keys
+``error_code`` and ``user_message`` and ``message_type`` into camelCase.
+
+.. _PaymentApiService: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/sagas.js#L34
+.. _transformResults: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/utils.js#L158-L172
+
+
+What about internationalization?
+--------------------------------
+
+Prefer using ``error_code``. It uses a pre-built component which should support
+internationalization like any other component with translatable strings in
+frontend-app-payment.
+
+
+For more information
+--------------------
+
+See `Feedback module's README and components`_, and especially the
+implementation in `feedback/data/sagas.js`_.
+
+.. _Feedback module's README and components: https://github.com/openedx/frontend-app-payment/tree/master/src/feedback
+
+
+Future work
+-----------
+
+Remove special error codes
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+There are certain error codes which receive special handling due to using
+pre-built APIs that do not follow the format of the feedback module in the:
+
+* `feedback/data/sagas.js`_
+
+    * ``basket-changed-error-message``
+    * ``transaction-declined-message``
+    * ``error_message`` in URL parameters
+
+* `payment/data/handleRequestError.js`_
+
+    * ``sku_error`` (Not an error code. See also `handleSDNCheckFailure`_.)
+    * ``payment_intent_unexpected_state``
+
+It may be beneficial for future refactors to minimize, deprecate, or
+despecialize the error codes with special handling.
+
+.. _handleSDNCheckFailure: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/feedback/data/sagas.js#L67-L79
+
+Remove special fields names
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When using the `Dictionary of field errors`_ format, certain key names in
+the dictionary of field errors will be transformed into another field name
+using a utility function called ``convertKeyNames``. See `utils.js`_.
+
+It would be good to deprecate this transformation by having the backend send
+correct key names.
+
+.. _utils.js: https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/utils.js#L185-L188
+
+Tame throw-catch bonanza
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+`handleBasketApiError()`_ and `handleApiError()`_ are thin wrappers around
+`handleRequestError()`_. The purpose of this wrapping is unclear. It may be for
+making stack trackes more local to the actual site of the error. Check to see
+if these wrappers can be refactored out.
+
+.. _handleRequestError(): https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/handleRequestError.js#L104-L116
+
+.. _handleBasketApiError(): https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/service.js#L12-L28
+
+.. _handleApiError(): https://github.com/openedx/frontend-app-payment/blob/1d631c51035eb8405ce9b03e0fa64a5a6e386268/src/payment/data/handleRequestError.js#L104-L116
+
+Standardize use of transformResults
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Fix inconsistant use of `transformResults`_ in `payment/data/service.js`_.