How do I arrange a stack of GUI elements?

I want to put one box over another. How do I do this?

Dave A's answer is definatly more precise (and technically the "correct" way to do it) but if you fancy less scripting it's also worth noteing that it can layer itself from "top of GUI script = bottom layer of GUI" so if you were doing a life bar you would 1st need the empty life bar GUI then the full life GUI then the life bar frame, do it in this order and they stack fine.