For your convenience, the library has been cooked down to two files: webp-on-demand-1.inc and webp-on-demand-2.inc. The second one is loaded when the first one decides it needs to do a conversion (and not simply serve existing image).
Copy webp-on-demand-1.inc and webp-on-demand-2.inc from the build folder into your website (in 2.0, they are located in "src-build"). They can be located wherever you like.
Create a file webp-on-demand.php, and place it in webroot, or where-ever you like in you web-application.
Here is a minimal example to get started with. Note that this example only works in version 1.x. In 2.0, the require-for-conversion option has been removed, so the procedure is different.
<?php
// To start with, lets display any errors.
// You can later comment these out
error_reporting(E_ALL);
ini_set("display_errors", 1);
require 'webp-on-demand-1.inc';
use WebPConvert\WebPConvert;
$source = $_GET['source']; // Absolute file path to source file. Comes from the .htaccess
$destination = $source . '.webp'; // Store the converted images besides the original images (other options are available!)
$options = [
// Tell where to find the webp-convert-and-serve library, which will
// be dynamically loaded, if need be.
'require-for-conversion' => 'webp-on-demand-2.inc',
// UNCOMMENT NEXT LINE, WHEN YOU ARE UP AND RUNNING!
'show-report' => true // Show a conversion report instead of serving the converted image.
// More options available!
];
WebPConvert::convertAndServe($source, $destination, $options);