You can google for just about any paper model there is, and if there isn't one already out there try to find the 3d file and unfold it in pepakura designer.
After you have the model printed, glue it tab for tab.
After that, buy some Bondo auto body filler, and Bondo Fiberglass resin, these chemicals will harden and make your pepakura feel like a finished product.
I mixed the bondo auto body filler and resin in about a ratio of 2-1, this makes the bondo much smoother, I used this mix to just slush around inside my mask (see pic). Sand down any rough edges, I used a mouse power sander, a dremel is more effective for those tight corners.
I used some craft foam for the detail on the forehead, just glued on with hot glue.
I painted it with spray paint, then used one of those cheap plastic brushes that come with a child's paint kit to texture the mask.
I then covered the inside of the mask with cotton velour, and put some foam under the velour to have it mold to your face a bit better.
All done! You can add straps or add horns or what ever you want!
Sorry i couldn't make a full picture tutorial, I didn't have enough pictures from start to finish of the project. Next time i make a pepakura i will take lots of photos!