Using the AS3 Performance front brake disc guard is easy. First, replace the front wheel spacer with the mounting bracket of the guard. Then, attach the guard to the bracket. Make sure it's firmly in place. When riding, the guard will protect your front brake disc from rocks and debris. To keep it in good shape, clean it regularly with a mild soap and water solution. Don't use abrasive cleaners as they may damage the plastic. Check the mounting bracket and the attachment points occasionally for any signs of wear or looseness. If you notice any issues, you can easily replace the guard as replacement parts are available. This guard will work well for your 2021 Rieju MR 300 Racing, Pro, or Ranger model, so enjoy a safer and better - performing ride.