Using a 15mm open end wrench, rotate the tensioner pulley counter clockwise grabbing the center bolt on the pulley wheel.
Slip the belt off of one of the closest pulleys and slowly release the tension on the tensioner pulley.
Before installing your new belt it is always a good idea to check all of the pulleys and make sure none of them are damaged and are free of any fluids or debris.
Installation is reverse of removal.
Route the serpentine belt according the correct diagram above for your engine size. Route the pulley around all of the pulleys accept the closest one to the tensioner pulley.
Maintain tension on the belt and rotate the tensioner pulley enough to slip the belt over the final pulley.
Double check all of your pulleys and make sure the belt is correctly aligned on each of them. Check your belt tension and make sure there is n more than 1 inch of play. If there is you may want to consider changing your tensioner pulley.