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Rotate the Tensioner pulley releasing the tension on the belt and remove. Inspect all of your pulleys and make sure they are clean and free of debris or fluids.
Route the belt according the diagram leaving the tensioner pulley for last. Rotate your tensioner pulley and slip the belt back on.
Inspect the belt again to make sure it is aligned correctly on each pulley. The tensioner pulley can be rotated a little to give it slack if you need to make adjustments.
Resources:
Basics
Read about tensioner pulleys.