Set your bra out on the table, front side down. You might need to smush the cup down a little to get it to lie flat.
Place a piece of tracing paper over the cup and secure it with tape or straight pins.
Trace the shape of the cup onto the paper with a pencil. Remove the paper from the bra and lightly sketch a second outline, about a 1/4-inch larger than your original drawing, to allow for error.
Cut out the cup shape from the tracing paper, using the outer outline.
Take a piece of jersey knit material that's soft and absorbent. Remember, this is the lining you'll feel against your skin, so pick something comfortable that will soak up sweat without making you feel soggy. Fold the fabric in half to make a doubled-sided pocket.
Cut the fabric to size, using your tracing paper pattern. Insert both layers of the bra pocket into the appropriate cup.
Sew the bra pocket into the bra cup, leaving an opening at the top for you to slip in your prosthesis. Put the bra on with your breast form and look in the mirror. If needed, take off your bra and trim excess material if any is visible from the front of the bra..