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2001-2003 Ford Mustang 3.8L
Rotate the tensioner pulley counter clockwise to relieve tension and slip off the belt.
Install: rout your belt according to the diagram leaving the tensioner pulley for last. Again, rotate the tensioner pully counter clockwise and slip the belt back on.
Inspect the belt and make sure it is on every pulley correctly. You can rotate the tensioner pulley to relieve a little tension and adjust the belt.
Resources:
Basics
Read about tensioner pulleys.
How to check your belt