As I said, sometimes experience is everything. It really would take an experienced mechanic minutes to fit it. Unfortunately that can’t be easily explained. Sometimes the belt is twisted where it has been in the carton. Perhaps in your case, find a suitable drill bit, spend some time and concentrate on locking the tensioner in position. I would think you’d have to get the tensioner almost to it’s fully retracted position which will be just about where it will not retract any further. Then fit the belt, take up the tension, remove the drill bit and release the tensioner. Don’t get your finger between the belt and ANY pulley.