Depending on the type of mower, there should be a couple of idler pulleys just below and in front of the PTO pulley. I'd check first of all which direction the PTO pulley spins. Then, by looking under the deck and spinning (by hand) the blades, see which way the belt to the PTO needs to move. That will tell you which way the belt needs to go on the PTO pulley. The two idler pulleys keep the belt tensioned on the PTO pulley, so the belt needs to go under them. You may have to loosen the adjustment bolts on the idlers to allow the big belt to be replaced. After you replace the belt, re-tension the idlers.
Clear as mud?
Hope this doesn't steer you wrong! Good Luck!