The goal is to release the tension applied by the spring-loaded tensioner.
I use extended sockets placed on the bolts of the tensioner pulley and tensioner base, then use a crow-bar or the like to apply leverage to lift the pulley. Be sure to keep the lever as close to the pulley and base as possible, so the sockets won't slip off the bolts.
Take things slow and easy, keep fingers and other mashable body parts out of the way, and only release enough tension to slip the belt off the alternator pulley.
Before removing the belt, make sure you have the belt path committed to memory, sketched out on paper, or have located the routing diagram - typically stuck on the radiator structure support, but it may be long gone given the age of the truck.
Route the new belt around all the pulleys, with the exception of the tensioner. Apply just enough leverage to lift the pulley high enough to slip the belt under. Again, watch out for your fingers, should the lever slip.
When changing a fan belt, it is a good idea, if circumstances permit,
to inspect each pulley and make sure there is no build up of any string
or similar crud.
Check each and every pulley to verify that it is rotating without any
Make sure that all the pulleys are in line and that the idler pulley is not
moving out of alignment.
This is just little stuff but it can save problems.
Did this with mine when I changed my belt. My tensioner pulley wouldn't even move. Bearing was shot. Half the pulley was polished so good from the belt that I could see myself in its reflection. Definitely a good thing to check. Also, diagrams are important, more important than I thought. I routed the belt how I "thought" it should go. Double checked that all the rib/flat sides lined up on the pulleys and everything. Then, I tried putting the belt on the last pulley for about 30 minutes without success. Finally I looked at the diagram one more time and it went on in two minutes. Felt like such a moron.
Live and learn the hard way or just read the directions
95 F150 XLT, 5.8L EFI, 2-wheel drive, k&n intake, magna flow exhaust, chip programmed with a 91 tune, custom lift with 86 F150 i-beams, dual fox racing shocks up front, deavers in the rear, 35x12.5 BFG All-terrains, ford racing posi
"Do it in the Dirt!"