I got the idea for this mask from this guy, earlier this year:
Step 1: Paper mache a balloon
I ended up using two balloons, so if the balloon color changes, that's why. One popped while I was taking a break.
Good instructions for paper mache-ing can be found herehere
Cooking the paste really works- I never did that when I was a kid, but things worked. Cooking the paste works BETTER.
Step 2: Attach cardboard fins and such
I figured i couldn't sculpt the whole fish out of paper mache, so I cut some cardboard pieces to mold to the shape of the fish that I wanted.
Wait until the paper mache dries for a day, then I used hot glue to secure the fins.
After you're FIN-ished (sorry, couldn't resist), use more paper mache layers to cover the attach points.
Step 3: Mark and cut the holes
This point is very crucial - you have to make sure that there are no serious weak spots on your paper mache-ing. This is critical because the paper will have to withstand some abuse from people patting your head, spilling drinks, people trying your head on, and others.
One way to check is to hold the head up to a light, or to put a flashlight on the inside and look for light spots. I made the mache approximately a 1/8th of an inch (2-3mm) thick.
Also, you're going to be making cuts to the mache, so you'll inherently weaken the overall integrity.