API

I can show you were to look if you are a PHP developer and know how WordPress plugins are built. Unfortunately, I haven’t created any guides yet.

Take a look at http://svn.wp-plugins.org/achievements/tags/3.3/includes/extensions/inviteanyone.php

Achievements works on WordPress’ actions. So, in your custom form, find or add a call to do_action in the appropriate place, and get the name of that action. You’ll need to set this in $this->functions in the constructor.

Essentially, what you need to do is duplicate that class, and set parameters to suit.
You’ll instantiate your class through a function hooked to dpa_init. e.g.

function chris_achievements() {
require( ‘your_file.php’ );
}
add_action( ‘dpa_init’, ‘chris_achievements’ );

Have a go, it’s pretty straightforward. If you get stuck, pastebin it, and let me know what the precise difficulty is. Good luck!