there are a few bolts that you will need to loosing and tighten.
a 19mm i think to turn the power steering pully (you will notice two slots on the front of the pully), turn it until the slot gives access to the 14mm (i think) bolt which keeps the power steering in place, just loosen that a lil' bit.
now, on the right side of the power steering (facing the front of the engine) you will see another 14mm bolt (which is pretty long), that is your tightener. just tighten that until the belt is, well, tight

don't over tighten though, feel the other belts to see how they are while you are at it.
once that is tight, tighten the power steering bolt, start it up and see if it fixed the problem.
if your belts are aged (cracked, dry, faded), now would be a good time to replace them.
-Dan