I saw some shark sweaters like this swimming about the internet, and, as usual, I opted to DIY instead of buy. I made my sweatshirt from scratch, but you can easily do this revamp on an old grey sweatshirt, too. Unfortunately I didn't take any photos while making it, but it's a pretty simple process, so hopefully I can explain it without pictures. :)
Draw or print a template for your shark jaw. Decide how large you want it to be on your sweater.
Cut out the inside mouth area from red fleece and the teeth from white felt. I used some iron-on vilene on the felt just to give it some support so it would keep its shape better. Make the inside mouth area only a tiny bit smaller than the entire jaw - the edges must be covered by the teeth, but must not stick out.
Tack your inner mouth and teeth together so that you have your complete jaw. Tack it onto your sweatshirt.
Machine stitch around the inner and outer edges of the teeth, thus sewing the entire jaw, including the inner mouth, to the sweater.