How do I put on a deck belt on a john deere sabre riding lawnmower model 1646

I have a 1990 john deere sabre riding lawnmower with the model number 1646.The deck belt has come off and I can't get it back on ?

Hi,
If need a picture, here it is:
Peace,
Clarke
PS The shield on the left is a belt guide, also. If the shield was off and the belt came off with the deck running (usually at time of disengagement) it has probably torn fibers internally and needs to be replaced..