Does anyone have a diagram on how to replace the drive belt for a 18 hp 46" hydro Sabre by John Deere?

Hi! gigglesrus, welcome to AMHD :)

Do you have any model numbers for that sabre?

Try this thread especially the first, and third.
askmehelpdesk.com diagram-install-drive-belt-john-deere-185-hydro-

Hi,
It is virtually the same as the STX; always under tension and virtually no guides. See below
Peace,
Clarke