Let's see if I can describe this clearly. Standing behind the deck, this puts the tensioner pulley slightly to left of center. You need to have the left gearbox mounting bracket off. The belt goes around the left pulley, over the near-side of the tensioner, around the center pulley and off the front of the center pulley to the right pulley, around the right pulley to return to the left pulley.
Helps a whole lot to release the tension adjust nut till almost off. Remember or mark the original place with a Sharpie marker. Reattach the left gearbox bracket and restore tensioner nut to original position.
That should do it, unless I was standing in-front of the deck, then everything is bass-ackwards. Pretty sure that is it. Basically the belt make a triangle and the tension pulley puts a jog in that side that runs up between the gearbox bracket and the gearbox pulley.