We are in the process of rewriting  help pages for paypal button but for now see http://help.wildapricot.com/display/DOC/Inserting+HTML+forms and for other specific paypal button codes you would need to use go here https://www.paypal.com/IntegrationCenter/ic_standard_home.html.  I am not sure I quite understand what you are trying to do ...

Hi, Is there a way to change the custom and required fields for the member signup form and the donation form separately? For the donation form, I want to add the ability for people to select what type of donation they can give, such as general donation, or event sponsorship etc. Then with a custom field I can have donators specify which event ...

Alec, you are totally right. We plan to add the same form customization to donation as we already have on the member application and event registration.