Can anyone give me a detailed escription on how to make UIs?

Here:

http://halopwner117.googlepages.com/createui.htm

If you want custom music, use this little script that I made up

(script startup music
	(begin
		(sound_looping_stop sound\music\title1\title1)
		(sound_looping_start "[your music]" none 1)
	)
)

It isn't the official way, but I couldn't find the official way anywhere (probably you have to edit some tag in Guerilla). But, it works, with minor bugs.