Step 1: Make the torso
Make a tube which goes around your waist up to under your shoulders. this part is going to take some of the weight of the costume, so it should be snug at the waist and sit on your hips. It's simply as a tube of foam, glued together with a 15cm overlap at the front or back.
Step 2: Shape the head
Whilst wearing the waist section and the head section, cut out some armholes and glue the two sections together.
Now you should be wearing a big tube, and you need to shape the head. Basically you need to cut it like a bishops hat (i've posted a pic of one for example) and glue the seam together at the top. I made mine so the front of the head is flat/vertical, and the back came forward to meet the front (you can see what i mean in the diagram). That's the opposite of what a shark looks like, but otherwise i think you'll find your head is poking out of the shark's mouth like it's eaten you feet first (which may also look cool!).