from wikiHow - The How to Manual That You Can Edit
A serpentine belt on this particular model car, installed several on other models and they are basically all about the same.
Purchase the correct belt for your vehicle at your local auto parts supply. You will need to tell them which engine you have in your car and also which accessories, such as power steering, air conditioning, etc. Each accessory means added length to the belt. Sometimes if you are in a pinch you can purchase the shorter belt and route it around the a/c pulley for example, but this does not always work.
Locate the belt tensioner. It may be a spring loaded arm or it may be a pulley that rides up and down on a threaded shaft. The kind on the threaded shaft should have a set screw to hinder vibration from loosening the tension. Most have a pulley on a spring loaded arm. Once you move the tensioner the old belt can be easily slipped off and a new one installed. Once you relax tension on the tensioner it automatically tightens the belt. Keep in mind that there may be a cover that needs to be removed to access the belts. It will be small and easy to remove.
Note the routing of the belt. If the belt has broken, then there may be a diagram on a decal under the hood or in the owners manual. You can also check out a Haynes or Chiltons auto repair manual in your local library.
Release tension on the belt by either loosening the set screw and then turning the threaded shaft, or levering up the spring loaded arm with a wrench (there should be an octagonal head that you can grip with a wrench or socket). Some tensioners have a square hole where you can insert the head of a 3/8 or 1/2 inch socket wrench or extension.
Remove the old belt, install the new one and retension. The spring loaded kind will automatically tighten the belt (if it does not, you have the wrong belt or missed a pulley when you installed it), but the shaft kind should be tightened until there is about 1/4" of deflection between pullies.
Tighten the set screws and replace any covers removed and you're all set.
Your library will have a manual that will have illustrations to guide you through the entire process
Your belt tensioner should have indicators that will inform you when the belt has been stretched to its limit. It will usually be two dash marks and an arrow.
Some cars will have more than one belt. Each belt will have its own tensioner.
Let your engine cool before starting repairs, especially if the belt that turns the radiator cooling fan has broken.
Never attempt repairs on an engine that is running.
If you have spare parts when you're finished, you've done something wrong. Go back and do it properly.