The best beginner guide for using the UIBuilder is this post:
http://www.eq2interface.com/forums/showthread.php?t=858
The only thing I would change is-- you only need to copy eq2ui.xml to your custom folder. You don't need to copy everything.
Other than that, just start with a simple window and try a small change. The documentation for the UIBuilder is really poor but you can find the answers to most things by searching the forums here.