[Zope-CMF] Customizing RegistrationTool/Membership sign-up in Plone

[Travis Milum]

Hello, I'm new to Zope/Plone/Python and I'm trying to customize the  
join_form and user registration process to include redirection to a  
payment site, a object to handle the passback data and finish the  
registration.

So, I've done tones of reading, searching and more reading but haven't  
found any good documentation of the user registration process.

I was hoping someone here could help out.

thanks for any help
Travis