How do you change a serpentine belt on a 91 Toyota pickup V6 3.0?

You'll notice an idler pulley. One that does nothing more than apply tension on the belt. Loose the front bolt of this slightly, then access the bolt that appears to go into the backside of the pulley and loosen it slightly to reduce tension on the belt. When installation of the belt is complete, tighten down the tensioner bolt till the belt is not overly tight, about 1/4" of spring is good to avoid slippage. Then tighten the lock bolt on the front of the pulley.