I need instructions on installing belt that drives the blades on John Deere GT 225

Hi,
Could be 42," 48," or 54" (or 44" Piranha - unlikely for this or 54") and the second and third are virtually alike.
There is an arm that must be unlocked on the left and swung rearward to remove the tension on the primary belt to remove it. Easiest thing to do is to unhook the tension spring for the secondary belt, install the belt and connect that spring last, making sure the belt is seated in all sheaves as you do it. Then reconnect the primary belt.