I always
1) Position the deck under the tractor.
2) Lower the undercarriage hanger unit as low as possible.
3) Slide the front bars of the undercarriage into the eyelets on the front side of the mower deck. If the spring loaded pins prevent this, then retract them all of the way and angle them in their slots (not in the holes) to hold their position, preventing them from expanding the spring.
4) Adjust the deck either forward or backwards so the hole slots on the deck undercarriage align to the holes for the spring loaded pins. Now pull on the spring loaded pins and adjust them to fit in the holes. Secure them in their holes.
5) Next, to install the belt, adjust the nut on the spring tension pulley as far as possible.
6) Route the belt onto the pulleys of the undercarriage and onto the deck pulley. If you are having trouble putting it on the deck pulley, get it on 1/2 the way and then rotate the pulley so that the belt slips onto the pulley.
7) Finally, adjust the nut on the tension pulley of the undercarriage so that the pulleys hold the mule drive belt at a right angle to the ground.
Itsa after 3 AM....hope I did not screw this up too much.
Cub Cadet 123