Remove the dust cover from the tensioner.
Rotate the self tensioner to release the tension on the belt and slip the belt off. You can use a small drill bit or allen wrench to stick through the eyelet to keep the tensioner retracted.
Route the belt around all of the pulleys leaving the self tensioner for last. If you have pinned back the tensioner remove your drill bit or allen wrench now while maintaining tension on the belt and pulley.
Slip the belt over the pulley and double check that the belt is on each pulley correctly.
It would also be a good idea to check the tension of your belt and the self tensioner and make sure it does not need replaced as well.