the belt tensioner has a pointer on it and you turn it until the pointer lines up with the FIXED notch in the body of the tenioner, ive had it before when ive replaced them that i set the tension but after turning the engine over twice the pointer has moved and the belt is over tight, some mechanics do over tighten belts, but TBH there is no reason to get it over tight as its not hard at all, post up how you get on and if it is over tightened mate