For a fun Halloween or school play costume that you can make at home, choose the shark fin. The first dorsal fin of the shark rides on the back, toward the centre of the fish. Sharks use their dorsal fins for stability. This fin pokes out of the water when the shark swims near the surface. Its ominous shape signals to swimmers that a shark nears. The dorsal fin's triangular shape makes it easy to replicate for costumes.
Draw a dorsal fin on a piece of paper to create the pattern. Add ridges to one of the long sides of the fin. Consider the size of the person wearing the costume when deciding on the size of the fin. This fin can be worn on the back or on the head. If the fin is for the head, curve the bottom to the contour of the skull. If worn on the back, draw the bottom straight. Cut out the shape.
Fold the fabric in half, inside out. Place the pattern on the folded fabric. The front edge of the pattern should rest at the edge of the fold. Pin the pattern to the fabric.
Cut around the pattern except for the folded side. Sew the side opposite the fold. Turn out the fabric so the sewn edge no longer shows.
Sew the bottom of the fin onto the plastic headband. Place the headband on the head of the person that will wear the costume on the head to see where it fits. Then put the fin on the head, over the headband. Put a mark or straight pin on the fin where it should attach to the headband. Mark the headband in the appropriate spots. Open the fin from the bottom. Fill with stuffing. Attach the fin to the headband by sewing into the fin and looping the thread around the headband. Do this to both sides of the fin.
Cut two pieces of elastic band the length that fits around the shoulder of the person wearing the costume on the back. Size the bands with the fin onto the back of the person. Use straight pins to attach the fin to the bands on each side. Be sure the elastic bands fit snug around each shoulder but loose enough to be able to take on and off without pulling the bands. Stuff the fin with stuffing. Sew the elastic bands onto the fins in four places, two on each side.