Park the John Deere 300X lawn tractor on level ground, set the parking brake and lower the mower deck into its lowest cutting position. This gives you maximum room between the 300X and the mower deck.
Disengage the PTO lever to turn off the mower blades. This creates enough slack in the drive belt for you to slip the belt off the pulley system.
Remove the belt guard that rests over the drive pulley mounted under the John Deere engine with a socket and ratchet. Place the guard off to the side.
Pull the belt off the drive pulley mounted on the John Deere engine, then pull the belt back towards the mower deck.
Pull the belt off the spindle pulleys and idler pulley on the mower deck. Place the new belt onto the pulleys. Stretch the belt towards the front of the John Deere 300X and place the belt over the drive pulley.
Place the belt guard over the drive pulley and secure it with the socket and ratchet.