Step 3: Plan the shape of your costume
I plan to use the balloon with the stem pointing straight up to create the shark's nose. As you can see from these two pictures, the balloon is round, bulbous, and the stem comes to a point at the center, but a shark's head is flat on the top, and the point of a shark's nose is upraised and ends at the flat top of the head. At least, that is what my scientific model shows me, and I have no reason to disbelieve its accuracy. So, we will need to add a flat top to the balloon in a roughly triangular shape, widest at where the eyeballs are and pointed at the nose, and add a cylinder to the base of the balloon to build up the shark-head to where the gills are. This will leave a gap between the stem of the balloon and the nose of the shark, but we will fill in that gap with pieces of tape.