Chumba, hit the FAQ link at top
Basically you just:
1) run the UIBuilder installer
2) create a directory for your modded UI
3) Copy eq2ui.xml from the default UI over to the new dir
4) Open UIBuilder then open the copied eq2ui.xml
UIBuider will automatically load all the other default files. Changed files will be saved to the dir eq2ui.xml is in.
If you wanna hack an existing UI like Nightfall, just open the eq2ui.xml from that directory
Once you're in UIBuilder, its fairly WYSIWYG. No need to know any XML unless you start hand coding stuff in Notepad.