I did the same thing as Drakah...just loaded up the UI Builder one day and started tinkering. I first made a "sandbox" UI directory, since I didn't want to screw up any of the default files.
I think the only real way to learn you way around the UI files is by a lot of trial and error. Tinker, save, copy to your custom UI folder, login to EQ2...if it's broken, you camp out and dig into the file again.
I almost never get my mods right on the first try, so all the logging in and out can be tedious.
(I never could figure out how to work the "Play" button on the UI Builder to test my mods offline without causing it to crash or freeze up. I'm not sure how other folks test their mods, so maybe I do everything the hard way, heh.)