The belt tool is crap, and mine broke while trying to take tension off the belt. Did a bit of research, and the 6.0 has a way easier way. Stand on the pass side of the truck like mentioned above, firmly grab the belt and pull up and out toward the fender. While pulling, there is a little tab that needs to be pressed in towards the driver fender (NOT TOWARDS THE REAR of the truck) and it holds the belt tensioner in full loose position. when you are done, simply grab the belt and pull and let go, in a sort of snapping motion and it will drop the tab back into place.
Hope this makes sense, but it is super easy once you get it, and no tools are needed. I wish I knew about this before fighting with a tool and the tensioner for about an hour and then giving up in frustration.