How do I examine the code of a Unity extension?

I want to learn how to make my own extension, but am not sure how to start.

http://forum.unity3d.com/threads/13115-GUIX-Demo-Released!

http://forum.unity3d.com/threads/47703-Public-Procedural-Roads-Paths-Tool

http://forum.unity3d.com/threads/47485-Public-Procedural-River-Tool

I really should have explained myself further in that other question. What I mean is you can download these, and you can see how they did their GUI on the editor... It'll help you get started atleast.

But

http://unity3d.com/support/documentation/ScriptReference/20_class_hierarchy.Editor_Classes.html

That should help most I believe. Sorry for the other vague answer =/.