Turn off the tiller. Locate the belt guard covering the drive belt. Remove the three screws from the drive belt, using a screwdriver. Pull the belt guard away from the tiller.
Remove the drive belt from the transmission pulley, using your fingers.
Remove the drive belt from the engine pulley. Slide the belt off the idler pulley and set it aside.
Install the new drive belt over the engine pulley, using your fingers. Slide the belt over the idler pulley. Slip the belt over the transmission pulley. Make sure the belt is seated properly on all three pulleys.
Install the belt guard over the drive belt. Insert the screws and tighten securely, using a screwdriver.