Using a 15mm wrench, turn the tensioner clockwise and slip the belt off the closest pulley.
Inspect all your pulleys to make sure they are clean and not damaged.
Route the belt according to the correct diagram. Maintain tension on the belt so it doesn’t slip off any of the other pulleys. Using your 15mm wrench again, rotate the tensioner pulley enough to slide the belt over the last pulley.
Double check all of your pulleys and make sure they are all on correctly. It would also be a good idea to check the tension on your belt to make or not if the tensioner pulley needs replaced as well.
Be sure to use all safety precautions when working on your vehicle.
Make sure the engine is off, the car is in park and you have applied the emergency break.