My belt came undone the other day, and was able to put it back on by loosening the bolt that connects the tension pulley to the engine. I am pretty sure this was not the way to do it, but i could not move the thing otherwise. Does anyone know how to properly tighten this belt? Also, can an old belt be the problem also?
The belt is properly tensioned by the automatic tensioner. Loosening the tensioner, as you guess, is absolutely not the right way to do it.
The tensioner has a 3/8" square in it at the center of the pulley. You put a 3/8" ratchet in there and rotate the whole mechanism while you slip the belt on. The bolted on part is a giant spring which provides the tension.
The real thing you need to figure out is why the belt came off to begin with.