Turn off the mower's ignition key to stop the motor and the mower blades from rotating. In addition, lower the deck to its lowest position with the deck lever.
Remove the bolts that old the right side and left side pulley covers onto the top of the mower deck with your wrench. Place the covers off to the side.
Draw a picture of the mower deck and how the belt routes around the pulleys.
Push in on the idler pulley, which is the only pulley with a spring-loaded bracket bolted to it. Lift the belt off the idler pulley and then remove the belt form the rest of the pulleys on the deck.
Place the belt onto the pulleys except for the idler pulley. Push the idler pulley inward and then place the belt onto the pulley. When you let go of the pulley it will put pressure on the belt to keep it from slipping on the pulleys.