I need to put the belt back on but can't figure it out! Either I've got it wrong (most likely the case) or the belt stretched out A lot. Thanks in advance!

Hi,
There are two options, get a Scotts manual online(assuming it is a Deere model) at:
http://dl.owneriq.net/2/2141e15a-36a6-9594-0166-6f10fba50f44.pdf
Or follow the link in my sig to the Deere deck belts.
Peace,
Clarke