Hi Auroz!
Did you unzip the UIBuilder in your EQ2 folder? That's the most recent one, dated 11/18/2005 and the doc is dated 3/4/2005.
The best beginner guide is this one from tonyis3l33t:
http://www.eq2interface.com/forums/showthread.php?t=858
A few other points I would add:
1. Copy the file eq2ui.xml from the Default folder to your custom folder if you don't already have one. (You don't need to copy everything like the guide says.)
2. Make a copy of your custom UI folder and only work on the copy with the UIBuilder. I just CTRL drag the folder down until a plus symbol appears.
3. Zonx actually recommends making one more copy of your custom UI folder. The reason is that when you save in the UIBuilder it saves a huge number of files, many more than you want to. It also doesn't save some of the files properly like the EQ2MAP files and the Infocenter files. So, you'll have your normal custom UI folder that is stable, the second custom UI folder that you're working on new UI pieces, and the third custom UI folder that is for opening and saving from the UIBuilder. After you save in the UIBuilder and it saves a ton of new files, just copy the xml file you want from folder 3 to folder 2. If you want to test out the changes in the game, point your eq2.ini at folder 2 and try them out.
4. Start out with a very simple change and work your way from there.
5. Download other custom UI pieces and look at how other people have changed the default UI.
6. If you can't figure something out search the forums or just ask here.
If you want to edit the xml files directly with a text editor I recommend
Notepad++ with the xml plugin. I only use the UIBuilder to visualize things and test some scripting but I do any actual editing in Notepad++ to avoid mess the UIBuilder makes when it saves.
Let us know if you have any other questions!