Find the serpentine belt routing diagram. Many cars come with a sticker in the engine bay that shows exactly how the serpentine belt is supposed to be routed through the various pulleys on the engine. If your vehicle does not have a sticker, you will need to find another way to remember how the belt should be run:
Look in the owner's or service manual for your specific year, make and model vehicle for a diagram
You can also take a picture of the existing belt with your cell phone to use as a guide when installing the new one