how do you tighten the belt tensioner?

 

It depends on what is loose. If the tensioner pulley is not pulling the belt tight enough (within the arrow marks on the clockspring housing) the tensioner is faulty. If the tensioner is on the high side then the belt is too loose.

If the tensioner is wobbling around (it's held in by two bolts) then obviously something is broken, be it the bolts themselves or the tensioner.