so..
you should to
1. download developer tools for version AbanteCart (just copy
this folder into your extensions directory) and install it.
2. create your own extension via admin menu and add some storefront controller in the edit form. This controller will be process you form data . Also do not forget to set hook file of your extension. You will need to change some basic page in admin via hooks... for ex form manager edit from page.
3. Also you need to set tpl for your storefront controller.
4. install and enable your own extension and start to develop