Disengage the blade clutch by moving the clutch lever to the neutral position.
Remove the bolts that secure the deck cover onto the top of the mower deck with a wrench. This exposes the pulley system for the mower deck.
Remove the cap screw that acts as a belt guide. It mounts to the idler pulley mounting bracket. Use an Allen wrench to remove the screw. The idler pulley is the only pulley mounted to a spring-loaded bracket.
Draw a diagram of both belts that you see. The belt routing around the top groove of the pulleys is the long blade drive belt, and the belt closest to the deck housing and riding on the bottom of the pulley is the short blade drive belt.
Remove the long blade drive belt from the pulleys on the mower deck and remove the short blade drive belt from the pulleys on the mower deck. When you pull the belts off the pulleys, push the idler pulley away from the belt to release belt tension.
Place the short blade drive belt onto the pulleys and place the long blade drive belt onto the pulleys. Follow the diagram you drew. Again, when installing the belts, push the idler pulley away from the belts. After installing the belts, let go of the idler pulley.
Place the cap screw back into the idler pulley arm and tighten it with the Allen wrench.
Place the deck cover onto the mower deck. Secure the cover with the bolts and wrench.