Loosen the belt tensioner. Make sure that the belt can be removed easily. Replace the belt with a new one. Check that belt is clean before installing. Clean the pulleys if you have to. Tighten the belt tensioner to set proper tension.
The power steering unit and the alternator are both driven by the serpentine belt. Loosen the tensioner pulley and the serpentine belt will become loose. Put the new serpentine belt on and tighten the tensioner pulley.
Use a socket and ratchet on the 1991 Astro vans serpentine belt tensioner pulley to pull it to the side, letting tension off of the belt. remove the belt and pun the new belt on. Release the tension to hold the new belt on.