How do you tighten the belt on a L110 John Deere Tractor Mower?
It keeps poping off the pulley

Hi,
You do not tighten the belt.
I it has been derailed multiple time, suspect that th cords within the belt are torn and it cannot ride straight. You did not sy whether it was drive or deck, but it does not matter much since both will do it.
Peace,
Clarke