I would highly recommend getting a friend to help whenever you have to try the pieces on for fit, it'll save a lot of time. I'm the kind of person who visits instructables a lot and thinks that spending the week before Halloween building a large and impractical costume is a better idea than going out and talking to people so consequently I have no friends to help out.
All the proportions and patterns are designed to fit a skinny person like myself, not much of a meal for a shark! YMMV.
Step 1: Stuff You'll Need
A large sheet of 1/2 inch thick white "Zotefoam" type material for the shark's belly.
A small sheet of 3/4 inch white "Zotefoam" type material for the shark's muzzle & teeth.
Zotefoam/Plastazote -type foam sheet is a semi-rigid polyethylene foam much stiffer than foam rubber. It's the kind of foam you find in camera and gun cases where a precisely-fitting recess is cut out to fit the item to be protected. It's also used industrially as flexible insulation board. If you hunt on Google you'll find suppliers on the web, it's light so not expensive to ship in large sheets. You could probably just about get away with some form of cardboard box material if you can't find/afford this foam. I happened to have a sheet lying around that I've known for years would come in useful one day...
Some 3/4 inch wide nylon webbing strap and plastic clip/buckle to make a belt.
Some 3/4 inch wide elastic and another plastic clip/buckle to make a leg strap and some hinges.
Gloss black self-adhesive signmakers' vinyl to make the shark's eye.
Ordinary A4/US Letter size office copier paper for the small, paper maquette sharks.
Large sheets of paper/cardboard for the full-size patterns. Anything you can draw on with a pen will be fine. I guess a cheap wallpaper (lining paper) roll would be fine, I had a large roll of 610mm wide plotter paper from my graphics work.
"Evo-Stik" -type impact adhesive. A kind of solvent-based flexible rubber glue. I think "Cow Gum" may be a simlar US equivalent but I'm not sure on this. You coat both surfaces with a stringy, messy, snot-like glue and then allow it to dry to the touch for 15 minutes or so before pressing the surface together for an instant (non-repositionable) bond.
A hot-melt glue gun is useful too.
Low-tack masking tape
Red, white and yellow artists' acrylic paint
Needle and thread to attach the buckles, you could get away with either of the glues and skip this but I like to know a costume won't let me down in a crowded nightclub where it'll get quite a beating.
1" wide self-adhesive strong velcro tape (white if possible).
Very sharp craft knife with lots of spare blades. IMPORTANT - don't skimp on new blades when cutting the soft foam. I used a snap-off blade boxcutter type of knife.
Scissors. Don't run with them.
Step 2: Make Your Shark Patterns
A major factor in the size of the shark relative to your body is that it has to have a large enough mouth to wrap around you without the body looking too short and stumpy. It will probably have to reach right up to your chest rather than sensibly around your waist - this costume is about fun and effect, not comfort and convenience!
The shark's grey back is made from two identical halves, one paper pattern will reverse to make both. To save foam (I didn't know exactly how I was going to do everything at the time) I cut the tail out separately but you could include it in the body pattern.
It turns out I'm about 6.35 times taller than a 1970s GI Joe doll so once I had a correctly-proportioned copy paper shark I simply took it to pieces and scaled them up 6.35 times on to huge sheets of paper.
Although the first couple of paper sharks were drawn freehand, once I was satisfied with the shape and size relative to the doll I flattened out the pieces and scanned them into my computer. I then traced out a neat version in Adobe Illustrator - I've attached it as a SVG file for anyone that can use scalable vector files. I scaled it up in Illustrator and drew it fullsize using a large plotter. If you don't have a vector application then open the file in your web browser - you could print it at A4 or US Letter and then scale it up, either drawn by hand using a grid or maybe something like an overhead projector. It's a simple shape so you could simply draw it fullsize by eye. The gentle curve along the top of the back (where the dorsal fin will go) is the most important bit as this gives the 3-D shape to the shark's body when the halves are glued together.
Repeat the drawing process for the two pectoral fins and the dorsal fin. You could include the tail at this point but I haven't included a plan for that as I measured up and drew it by hand once the main body of the shark was glue together so I could get the shape and size I wanted. If you have a big enough sheet of foam you could simply include the tail as part of the main body halves but I had to place it on the foam separately.
Finally, draw out the pattern for the u-shaped 'muzzle' area.
Step 3: Cutting & Glueing the Shark's Back
Using a craft knife with brand new blade to carefully cut through the foam. Keep changing to new blades if you feel the soft foam starting to catch and tear. Don't cut your fingers off as you'll need them later. It may take 5 or 6 careful cuts along the same line to get through the thick, soft foam. Take your time - don't try and press hard to get through all at once. Also remember to make sure you have a sheet of cardboard beneath the foam to protect carpets/tabletops/floors as you cut through. I use a cutting mat and just reposition it beneath each length of cut. The more rigid 'Zotefoams" cut much more easily with the same technique.
Repeat the process for the two pectoral fins and the dorsal fin. Then again on the 3/4 inch white 'Zotefoam' for the muzzle.
Coat the top edge of the two main body halves with impact adhesive, allow to touch dry and then carefully press the halves together, a few inches at a time. Leave the tail stubs unglued as they will point upwards, not be facing each other like the rest of the seam. Make sure you keep a nice, straight seam and that the foam pieces are accurately lined up. The curves of the pattern will automatically pull the shark into a curved, 3-D shape as you bond this top seam.
When the two halves are joined you can add (impact adhesive again) the shark's 'muzzle' in the more rigid, white foam which will stiffen and round out the sharks nose and mouth area.
Carefully coat the attachment points for the 3 fins with impact adhesive, and the joining faces of the fins themselves. If you're not painting the body, any careless glue patches will show. Once touch dry, attach the fins (two at the sides and the dorsal fin at the top) the same way you joined the body halves. Carefully and the right way around!
Bend the two tail stubs upwards and impact glue them inside where they touch. Make up a paper pattern by eye for the rest of the tail and cut the shape twice out of the grey foam rubber. Use impact glue (or maybe carpet fitters' spray glue if you have any) to join the two tail halves and bond them on to the tail stub on the body. Bevel the leading edges of the tail and dorsal fin a little with a sharp knife if you're feeling confident, it's very tricky on the soft foam.
I had originally planned on having a full tail but decided to only go with the (wearer's) front half so that it's possible to stand back against a wall to get out of the crush in a crowded nightclub. The double thickness gives enough strength to take an evening's beating from being trodden on in crowds and also adds a counter-weight to tilt the costume in tight against the front of the legs to get a nice, tapered shark look.
Step 4: Making the Shark's Belly
I did a lot of test fitting and then used a heat gun (careful not to melt the carpet, burn your fingers, etc) to curve the two sides so they wrap closely around my body and fit inside the shark's main grey foam body. A lot of trial and error bending by hand, holding the curve, heating and then holding the shape till the foam cooled. If you don't get it hot enough it won't hold the curve and if you get it too hot it will blister, melt, burn, give off poisonous gases and eventually burn your house down with your unconscious body inside it. There is a slight taper from top to bottom, much like myself ;).
Step 5: Teeth!
Trace out the patterns on the 3/4" thick white 'Zotefoam'. Settle down for a long and careful cutting-out session with new blades. You may be able to see from the photos that I cut with a strong bevel (slanted cut through the thickness of the foam) on the teeth so that the remaining foam shows bigger teeth shapes cut out on one side than the other.
Mix some artists' acrylic paints to a bright shade of fleshy pink (try a combination of red, white and yellow for flesh) and paint in the gums. I didn't bother with neatness and allowed the rough edges of the cut foam to give a bit of texture.
I allowed the acrylics to dry overnight as that's just how the timing fell that day.
Next use the same heat gun technique to curve the teeth to fit the upper and lower jaws. It's easy with the longer set of teeth to fit the upper jaw in the grey foam body but requires a lot of careful and complicated bending in two directions to match the shorter lower set of teeth to the shark's belly. I actually had to unglue the lower teeth and cut a sharper curve into the gum at eihter end to make it fit around my body properly after a test fitting. I also went back in with the knife and cut the outer 3 or 4 teeth on each side of the lower jaw progressively smaller for a meaner, more uneven look and to allow the belly to wrap closer around me to fit inside the shark's back. Remember the shark's back is worn on my fornt and the shark's belly is worn on my back...
Using impact glue on both surfaces as before, bond the teeth into the respective body halves. A central line marked on (the inside of) both the teeth and the body helps to line things up - then start sticking down from the middle outwards.
Step 6: Test Fitting, Hinges and Straps.
Will holding the shark's belly in position, mark where the back of your knee is on the inside of the white foam. We are going to put a hinge in here to make walking just a fraction easier. But not too much easier, convenient costumes are for wusses.
Now make a straight cut right across the belly at the knee joint level, cutting from the inside face only about 3/4 of the way through the thickness of the foam so the hinge won't show on the outside of the costume. Patience, a sharp knife and a steady hand are required. Then, equally carefully make a 45 degree cut either side of the original cut to take out a triangular wedge of foam right across the knee to allow the foam to hinge at the thin point.
The curved sides cannot bend back and forth so simple go back and cut right through the foam at the sides, stopping at the flat belly. I used a little hot-melt glue at the ends of these cuts to stop the foam tearing across the whole belly.
Cut 4 short lengths of flat elastic and glue them (impact adhesive) across the hinge point at the sides and on the flat of the belly to help pull the hinge closed again with each step.
Again holding the belly against your body at the right level for the shark's back to sit properly on your front, mark a point to add the waist belt.
I made the waist belt from a length of black nylon webbing strap with a simple plastic buckle at one end. You can get this sort of thing from B & Q in the UK, (Home Depot in the US maybe?) or any good hardware store. I melted the ends of the strap slightly to prevent fraying and glued and stitched the buckle on. You might get away with just hot-melt glue if you're not keen on needles.
Bond the belt to the shark's belly at hip level, using impact adhesive along the full length of contact. This has to support the full weight of the costume. Where the belt leaves the edges of the belly foam I added a little hot-melt glue either side of the strap to reinforce these stress points.
On the grey foam half of the shark an elastic calf strap is needed to both taper the shark's body in properly at the bottom and hold the lower half of the belly against your legs as you walk (shuffle comically like a penguin!).
Make up a strap of the right length, enough to fit around both your legs AND the sharks belly as it has to go around the outside. This time use white elastic, not nylon webbing, but the same type of buckle is fine. The buckle goes on one side on a short piece of elastic, just long enough to loop through the buckle and give an inch or two of glueing area. The long length of the strap is just elastic. I cut a diagonal end to help insert it in the buckle and used a tiny bead of hot-melt glue to stop the elastic fraying. Impact adhesive will join the elastic to the foam and to itself to hold the buckle. Glue the buckle and the strap close enough to the edges of the body to pull it in around you nice and close, but remember to place them inboard just enough so that the grey body hides the buckle. You can see the placement in the photos, about mid-calf height.
Step 7: Attaching Front to Back.
Using impact adhesive, carefully glue in these two extra pieces of foam, flush with the edge of the body like for the 'muzzle' piece.
Apply a length of self-adhesive white velcro to the 3/4" edge thickness of these two pieces, fold the extra 1/4" of velcro away on the inside. This has to support the full weight of the back of the shark. Use the soft 'loop' side of the velcro so it doesn't damage the soft foam rubber when being transported.
Hold the costume back in position on your body and get someone to mark where these velcro strips will sit against the shark's belly. Add the corresponding 'hook' side of the velcro at these points on the belly. I used two strips side by side to give a larger target when putting the costume on.
Step 8: And Finally...
For the costume party, first put on a bum-bag (fanny-pack) backwards so the pouch is to the front where you can get at it during the evening. This is somewhere to keep money, keys, etc.
Line up the shark's belly at the right height and strap it on your back with the waist belt.
Lift up the shark's back, line the velcro up and join it to the belly so it's now supported hands-free.
With enormous difficulty, or ideally a helper, bend forwards and loop the calf elastic around the belly and buckle it tight enough to hold the body in close.
Attend the party and get loads of appreciative laughter for such a great costume and prepare to be in everyone's photos of the night. Remember that reaching down inside the shark's mouth for money, etc will look like you are doing rude things to yourself. And vice-versa...