Before you get excited about there finally being a good replacement belt available... a lot of the vintage original belts are still going strong and may not need replacing at all. So, definitely check your belt carefully. "If it isn't broke - don't fix it."
This is the plastic, lugged belt for those who prefer them for aesthetics, or personal preference. The function and size are exactly the same as the black belts.
How to Replace and Install the Featherweight Belt
To replace and install the belt you will need to raise the motor up. To raise it, there is a screw on the back of the motor housing (seen from the front of the machine at the right side, behind the hull). Standing over the machine, with an appropriately-sized screwdriver in your right hand and your left hand holding the motor (you're leaning over the machine); you'll unscrew the screw just slightly and adjust your motor up a little bit so that it can loosen your current belt all the way. Remove the belt from around the motor pulley and handwheel and replace it with the new belt in the same manner, making sure that it is tucked into the correct position into the handwheel.
Lower your motor and tighten the screw on the motor housing bracket. You want your belt as loose as possible yet still have the ability to turn the motor pulley and handwheel when you step on the foot controller. (This is best tested with fabric actually under the presser foot.) If the belt is too loose; it will slip and the motor pulley will just spin and your fabric will not feed. If the belt is too tight, it will put unnecessary strain on the motor.
If you want to protect the finish on your machine; place a soft cloth between the screwdriver and the machine's hull or you can scratch the finish by trying to access that oddly-placed screw on the motor housing bracket.
To adjust your belt after installation, be sure to checkout the Featherweight Schoolhouse for a video tutorial.