The father documented the process on how he created the mask by taking photos as he made it. He then uploaded the photos on Imgur. Here's a repost of said photos. All photos and descriptions below are by Skissored via Imgur.
Made from Crayola Model Magic foam, I usually buy a big tub of it from Staples for about $12-$17 but I think Michaels and Wal-Mart sell individual packs for less.
Used a butter knife to get the lines.
Lay over a mannequin head to hold the shape and dry for a couple days, turning it over halfway through to dry both sides.
Ready to paint, I used acrylic I had around the house. Any brand of paint will do, it just comes down to personal preference.
The foam is still slightly squishy when dry so it will need a few layers of a hard clear coating to strengthen it. Painting first.
First layer of black paint to get the shadows in the cracks.
Dry brush over brown.
Then the green and gold.
The finished mask.