Is there a good guide to writing rlm modules

I am a newbie to Radius, and need to implement some function that is not easily done by any of the existing modules. So I want to write my own, and being a Java programmer I chose JRadius as well we FreeRadius. I wrote a module but it is not taking my acceptance of the user i the authorize step, but rather send a rejection to the post_auth step. Obviously I am missing something, so I thought that some documentation on writing modules would be good reading. Anyone know where I can find some? Thanks David