I can't help much from here, I have never worked on a John Deere garden tractor, but you will need to remove belt to blades 1st.
Do you still have the manual for tractor ? If so I think it might tell you how the belt goes on.
Now would be a good time to check all the pulleys, grab them and see if you can move them up down Sid to side and spin, if it is rough feeling or make a growling sound you need to replace them.
Hopefully someone who has replaced one will chime in, if no one does I'll do some digging and see if I can help.
Also be sure and go with a John Deere belt, the cheaper ones won't last very long.