It's easy. Get a new belt. Find the correct size socket for the bolt on the tensioner pulley. Use a breaker bar or 1/2" socket wrench and try to remove the bolt on the tensioner pulley. The tensionor will just spring upward loosening the belt. Pull the belt off of alternator and release the tensioner. Remove the rest of the belt.
There should be a picture on a sticker showing you the belt routing right in front of the radiator. Put the new belt on according to the picture (or if you remember how the old one was just do it that way). Save alternator for last. Do the thing with the tensioner again and slip the belt on alternator pulley. Make sure the belt runs under the tensioner and then release the tensioner.
That's all there is to it. It should take you about 5 minutes at most.
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351W 89 Mustang GT Convertible