Your doctor will take measurements to fit you for your knee brace. Sizing is determined by taking a measurement of the circumference of the thigh, six inches above the knee cap.
Each knee brace comes with detailed instructions on how to put it on. It is very important that you follow the exact sequence of inserting and tightening the straps. If you need to tighten any one strap after you have been wearing the brace, you will need to redo all of the straps in the proper sequence, not just the one strap that needs tightening. It is recommended that you work with a physical therapist or other healthcare provider and practice fitting your brace until you are comfortable with the procedure.
Most braces also come with specific instructions for that brace which should be followed carefully.
Most websites also provide fitting instructions specific to the particular knee brace if you are ordering one online.