Here are some instructions that should helps you.The belt tensioner is the part that applies the correct tension to the belt.
The belt is tensioned automatically. If the belt is slipping, either the belt has stretched beyond its serviceable limit or the tensioner is faulty.
Fig. Accessory drive belt routing-all engines
Fig. Accessory serpentine belt routing-Mercedes-Benz 3.2L engine
Inspect the drive belt for signs of glazing or cracking. A glazed belt will be perfectly smooth from slippage, while a good belt will have a slight texture of fabric visible. Cracks will usually start at the inner edge of the belt and run outward. All worn or damaged drive belts should be replaced immediately.
Fig. Exploded view of the drive belt removal