Determine the amount of fabric required by measuring the desired length and width.
Remember to allow for other clothing that will be worn under the costume.
Draw out a large rectangle with a rounded top. It should resemble the top of a banana.
Cut two pieces of this shape.
Cut four layers of fabric for fins. These can be long enough to accept arms or can be attached to the sides of the costume body.
Cut out a large oval in pink, or red, for the mouth opening, or you may wish to simply cut the entire mouth opening for the face.
Sew the fin, right side together; stitch along seam allowance. Turn inside out, press and repeat for the second fin.
Stitch around the entire body along seam allowance. Turn inside out. Press seams open.
Attach fins to sides. Cut openings for arms.
Sew the white mouth oval to the gray front at face level. Cut the opening to create jagged teeth in the mouth area.
Apply black ovals for the eyes over the mouth area.