Take off the old fan belt:
Take the old belt off of the pulleys. You will need a wrench or two to release the tension on the belt. Tension systems differ between makes, but the effect of each is still the same: They maintain a tight belt when in use, and they allow you to adjust the tension. In this case, you want to reduce the tension on the belt so that you can slide it off of the pulleys.
Get the right size replacement belt:
You'll need to get the right size replacement belt at any local auto parts store. It's very important to match the new fan belt shape with the old one. Take the old belt with you to the store to compare with the new belt. Sometimes the new belt is a little bit different length. If the new belt is a centimeter or so short, then that is OK. It will likely stretch out a little bit. If the new belt is a little too long, ask the salesperson to help you find a closer match. A longer belt isn't acceptable.
Install the new belt:
When replacing fan belts, route the new belt through the pulleys so that it uses the exact same path as the old belt. You may need to loosen the belt tensioner a little bit more in order to slip the new belt onto the final pulley. Refer to the belt diagram to check your work. After the belt is in place, tighten the belt using the tension adjuster on your car.
Helpful fan belt repair tips:
The belt should be taut, and should not squeal when the engine is running. To determine if the belt is tight enough, remove anything that could get caught in moving parts and turn on the engine. Turn on the air conditioner. If the belt doesn't squeal, then the belt is probably tight enough.
When you tighten the belt, be careful not to get it too tight. A belt that is too tight will put too much stress on all of the pulleys in the system and will cause one to fail over time.