Loosen the drive accessory's pivot and mounting bolts.
Move the accessory toward the engine until enough slack is created to remove the belt from the pulley.
Place the new belt over the pulley and move the accessory away from the engine until the tension is correct. You can use a wooden hammer handle, or broomstick, as a lever, but do not use anything metallic, such as a prybar.
It is better to have belts too loose than too tight, because overtight belts will lead to bearing failure, particularly in the water pump and alternator. However, loose belts place an extremely high impact loadon the driven component due to the whipping action of the belt.
Tighten the bolts and recheck the tension. If new belts have been installed, run the engine for a few minutes, then recheck and readjust as necessary.