Guidelines for developing mods?

Hello,
   I've been browsing around the site looking for a document or post that outlines guidelines / requirements for writing and installing a mod that will be compatible with WWF 9.05; I have just installed the Premium version of this product. Does such a document exist, or is the process just 'read enough source code to see how things work, then hack until your changes work'?

In case it matters, I'm specifically intersted in doing a Paypal mod, since according to the author of one that was released a while back there will not be a 9.x-compatible release of his mod.

Thanks,
Chris