Disengage the blade on your John Deere 214 mower and park the machine on a level surface. Set the parking brake and cut the ignition. Lower the mower deck via the lift lever to the right of the driver's seat.
Look along the center of the deck for the pulley the John Deere's drive belt is connected to. Find the spring loaded idler piece, which is the circular part directly beneath the spring and pulley that controls the drive belt's tension. There is a bolt at the top of the idler plate that must be loosened to adjust the mower's belt for proper fitting. Twist this bolt counterclockwise with a socket wrench to loosen it.
Insert a breaker bar into the square hole on the idler plate. Turn this bar clockwise to tighten the John Deere mower's belt until you feel resistance. Do not tighten the belt once resistance is initially felt.
Tighten the idler plate's top bolt back up with your socket wrench. Pull up on the lift lever to raise the John Deere's deck back up.