1. Get a eight to ten inch foam half-circle (can be found in Michaels or Joannes in the flower section). I tend to leave the plastic on, even wrapping in in more plastic, that way it'll be easier to remove the shell from the foam later.
2. Paper Mache the circle, anywhere from eight to fifteen layers, letting each layer dry. I make my own paste with flour and water, sometimes with white glue.
3. Once the paper mache layers are thick enough (around fifteen layers is recommended) and has fully dried, carefully pop the foam away from the shell. The easiest way I've found so far is loosening the edges first with a dinner knife, then stabbing it deep into the foam at a angle, and slowly pulling the foam away. There will be damage to the foam and the shell, but all shall be fixed soon. If you decide that the paper mache is still too thin, just stick the foam back into it and put a few more layers on. Once you're happy with the shell, coat the entire piece inside and out with Paper Clay or a equivalent air-dry clay. Give up to 24 hrs to dry and sand to smooth. Sketch the design onto the mask.
4. Cutting out the eyes out can be done with a scalpel, but it will be slow and a bit painful. A quick way to do it is to drill a hole with a power drill, then cut out the shape with a jigsaw. It can be a little tricky, but I can be impatient and I wanted those eyes out NOW! I've put in a few pictures of San's mask in progress as examples. I'm making more Deku masks in the future and I'll be sure to take more progress pictures.
5. To cut out a hole for the mouth I use a power drill again with a large circular bit so I can get a perfect hole. For the mouth itself I bought a small wooden mug from Joannes in the dollar bin, took the handle off and cut off the bottom so I had a tube. Then I superglued the mouth into the hole at a slight angle, making the mouth shorter on top. Once everything dried I filled the gap around the mouth and inside the eyes with paper clay.
6. Paint the mask with a combination of browns, black, yellow, red, and orange in a gradation from dark to light, the dark brown/black ending right below the eyes. I painted the entire mask a base shade of brown, then painted the black on top and drybrushed the paint downwards to create a gradation. Same happened with the yellow shades, allowing my brush to get nice and rough so it creates a nice, almost wooden texture and keeping the yellow visually interesting.
7. The eyes I made out of yellow celluloid with orange paint to make the "glow". Once you like the color and shape, glue into place on the inside of the mask. The leaves are silk leaves from Michaels and were a bit tricky to glue. One I found three leaves I liked that had the right color and shape, I took them off the fake plant and removed the stem backing. Now that the leaves are softer and easier to bend, I was able to glue them to the inside of the mask, then bend them while the glue dried so they could stand erect on top of the mask and not flop over.
8. Congrats, you're done! You can also glue a elastic strap to the inside of the mask or put a loop in so it can make a nice piece of wall decor. Thank you for looking at this and let me know if you have any questions!