One of the biggest problems I run into while wearing a rock climbing harness for tree climbing is that I have so many pieces of gear clipped to my belay loop the mess is nearly impossible to deal with. This modification pulls your lanyard attachment points to the sides of the harness and frees up room on your belay loop for your main climbing system. Now, I totally understand that adding lanyard attachment points to a rock climbing harness does NOT automatically make it a tree harness but I think this is one of the biggest distinctions between the two types of harnesses.
This tutorial does not require any sewing. All you need is your harness, 8-9 feet of webbing, and two rappel rings (this is the cheapest 32kn rated ring I have found: http://www.treestuff.com/store/catalog.asp?category_id=303&item=1684). You may have noticed that my personal modification has a buckle on it and this makes the setup much easier to use but this means I have to use BOTH rings at the same time. If I only use one of the rings and the un-rated buckle fails then I would fall to my death. But if you tie your webbing like in the video you do not have to worry about this! Now if I could just figure out how to add an adjustable bridge to my harness!