Pull the power cord out of the wall outlet. Move the washer away from the wall.
Locate and unscrew the screws holding the rear access panel in place. Lift the access panel off to reveal the motor and belt.
Locate the black rubber belt. Gently pull on it to see how much it moves. If it pulls more than 1/4 inch, it needs to be tightened. Make sure each end of the belt is connected to the pulley system on both the motor and the back of the tub.
Locate a nut on the motor near where the belt connects to the motor. Loosen this nut about halfway with a wrench. Do not fully remove the nut.
Tap the metal motor mounting plate gently with the wrench. The mount will move slightly. Continue tapping as the belt tightens. Pull gently on the center of the belt; once the outward pull is 1/4 inch or less, stop tapping the mount plate.
Tighten the nut. Replace the back access panel and screws.
Move the washer back into place and plug it back in.