Get on your back under the truck and use a 3/8 ratchet on the tensioner. Put the ratchet in the tensioner (there will be a slot for it as if it were a socket) and if I remember correctly you have to pull down on it to release the tension off (you'll figure that part out, it's a cake walk). When you release the tension, pull the belt off one of the pulleys and slowly release the tensioner. Once you've done that, remove the belt. Put the new belt in the way the picture on the header panel shows. Once you re-run the belt, you will have to put tension on the tensioner again so it goes over all of the pulleys. The easiest way to do this is to run the belt, put tension on the tensioner again, and have someone pull the belt over the last pulley. Once it's on, release the tensioner slowly and you're done.
Hope that's simple enough for you, welcome to R-F.