When purchasing belts, remember that the critical parameter that determines whether a belt fits or not is its length ("diameter"), width, depth and angle or profile in the form of a letter V. The shape of the belt must exactly match the shape of the pulley; Belts that don't fit exactly can cause noise, slippage and premature failure.
If you need to replace the belt, first loosen the adjuster, if equipped. The drive piece may also need to be loosened or repositioned towards the engine in order to be able to remove the belt. After removing the old belt, check the pulleys for dirt or anything that would prevent them from making contact with the belt. Fit the new belt carefully, remembering that it is still unused and therefore may be slightly smaller when slipped over the pulley flanges. Put the belt on the largest pulley first (this is usually the crankshaft pulley at the center of the engine base), then put on the smaller one.
A slight pull in the direction of rotation can help seat the belt. Depending on the belt you are replacing, it may be necessary to loosen or remove other interfering belts to get to the one you need. Once the new belt is in place, pull the drive unit away from the engine and tighten the set bolts. This work may sometimes require three to four hands; resort to someone else's help. Make sure that all previously loosened bolts are re-tightened.