The blade belt came off my Snapper Z1803ZK zero turn mower and I can't figure how to put it back on. Can you help?

Hi,
First let me say that the manuals are available online though you need different numbers. There should be a black on silver tag under the seat with a model number that is probably five digits and begins with "80..." There should also be a similar tag on the deck with its model number.
Snapper is now owned by Briggs as is Simplicity and their manuals have been "folded" together so without those numbers, it can get difficult and time-consuming.
I am going to assume a 48" deck beause of the horsepower, yes that "18" part of the number you gave.
Peace,
Clarke