This is a WordPress plugin, meant as a starting point for developers to tweak WP Offload S3 using WordPress filters.
Create a /amazon-s3-and-cloudfront-tweaks/ folder in /wp-content/plugins/ and simply drop the amazon-s3-and-cloudfront-tweaks.php file into it. Then go to the Plugins page in your WordPress dashboard and activate it.
Open the amazon-s3-and-cloudfront-tweaks.php file and take a look at the __construct() function. You will notice that all the calls to add_filter() are commented out. So, at the moment the plugin does nothing even though it's activated. To enable a filter, simply uncomment the appropriate add_filter() line.