To route the deck belt on a 42 LS45 mower, first, ensure the mower is turned off and the key is removed. Begin by placing the belt over the engine pulley, then loop it around the idler pulleys and spindles, following the mower's specific diagram usually found on the mower deck. Ensure the belt is seated properly in all grooves and tensioned correctly before reassembling any covers. Finally, double-check the routing against the diagram to confirm accuracy.