The dorsal fin is an iconic feature of the shark. As such, any good shark costume would be incomplete without a prominent fin; you might even wear the fin on its own in certain circumstances. A shark fin is a basic shape, and making one that's attachable as a costume piece is relatively simple, but it's important to take care to make the fin fit comfortably and look believable.
Draft a paper fin pattern on graph paper or newsprint. Start with a large sketch of a right triangle, then reshape the short side to have a dramatic inward curve. Give a slight outward curve to the outside acute angle as well. Cut out the pattern with scissors.
Cut the pattern from foam rubber. Pin the pattern to the foam rubber using straight pins, then cut the rubber around the outside of the pattern using a craft knife.
Fasten the two belts to your torso. Attach one belt at the broadest point of your chest if you're a man and beneath your breasts if you're a woman. Make sure both belts are snug but comfortable.
Pin the fin temporarily to the belts. Position the fin where you want it, then use safety pins to attach the fins where they meet up with the belts. You may need the help of a friend, or turn the belts around on your body and pin the fin in front. Check a full-length mirror to make sure the fin is where you want it.
Attach the fin permanently to the belts. Remove the belts from your body, then sew the fins to the belts where they're attached by the safety pins. Sew along the edge of the foam rubber using a whip stitch (over and around).
Spray paint the fin. If your fin needs extra stiffness, use extra layers of paint.