I have a Toro 6.5 HP GTS lawn mower, model #12OVHO. The belt has slipped off. After removing the blade and the cover, what else do I need to remove or loosen to be able to put the belt back on the blade's flywheel?

Lower the deck all of they way and disenageg the blade. Remove the belt from the engine to the deck. You may need 4 hands since there is a spring tension that needs to be retracted. With that belt gone you now move to the tensioner on the deck, pull it back. The belt should come right off. Most decks have two more pulleys than they have blades. One of the extra pulleys is stationary and the other is the tensioner.