Start with vegetable-tanned (also known as saddle-skirt) leather. I picked mine up at a local Tandy Leather. Get 5/7-weight leather (that is, between 5/64" and 7/64" thick). Thicker is OK, though you'll get more shrinkage (less soak-time may help that). Thinner won't harden as quickly, will be brittle and won't hold a shape very well.
You'll also need paint suitable for leather (fabric paint usually works).
Step 2: Cut pattern
Cut a mask pattern, allowing for about 30% shrinkage.
Step 3: Cool Soak
Soak leather in cool water for 10 minutes.
Step 4: Heat water
Heat a pot of water to 180Â°F.
Step 5: Hot soak
Soak in hot water for around 90 seconds. The leather will shrink, curl and thicken, and then start to uncurl. The longer you soak it the stretchier it will be at the start and the smaller, harder, thicker and more brittle the end result will be.
Step 6: Shape
Once the leather comes out, you have about 5 minutes to stretch and shape it before it becomes stiff. For a good face-shape be sure to add bumps for the bridge of the nose, eyebrows and cheekbones. If you have any dangly bits they can be twisted or braided, and they will harden into whatever shape you set them. Over the next 10 minutes, give it a pinch every now and then to make sure it stays in the shape you want, then let it dry overnight. By morning, it should be hard as wood.
If you decide now that you need to expand the eyeholes or change the outline you can use a drill or saw. Otherwise, you're ready to paint it, using a paint suitable for leather. I used several coats of "3D" fabric paint. Mardi Gras colors are green, purple and gold. Add ribbon-ties if you like. Have fun!