Well, it depends on what you're trying to figure out and how deep you want to get. The official documentation is pretty poor and nearly useless except for a few reference things. The best beginner guide is the one tonyis3l33t made a long time ago:
http://www.eq2interface.com/forums/showthread.php?t=858
The only thing I would change would be that I would only copy the eq2ui.xml file and not every default file.
After that I would start off with something very simple like changing the size of something or removing one thing, etc.
After that you have to start figuring things out. I really think we could use a wiki to organize all the information and help out UI modders but I don't know if anyone wants to step up and start one. The forums have a ton of information and doing a search usually gets what you want.
The way that I work, I only use the UIBuilder to see things visually and look at the xml structures. I just hate the way that it saves the files and saves dozens of files that you didn't edit. Every change I made is done with a text editor so I have better control of what the xml looks like. I use
Notepad++ with the xml plugin that makes things much, much easier.
You said you're working on the POI Finder which connects to the eq2map. The eq2map is likely the most complicated mod around, since it uses so many xml files linked together. I would start out by dissecting some of the simpler single file mods.