Harrison Krix of Volpin Props (a prop maker) has uploaded a video to Youtube demonstrating the process he performs to create a real world version of the Helm of Yngol.
More pictures and explanation are available at Harrison's blog. Some of the steps:
After completing the hammered texture, I attached the horn bases. These started out as vacuum formed styrene discs with acrylic rings glued to the helmet sides. After securing them in place, they got the same apoxie sculpt and hammered texture as the rest of the helmet.
The face plate section came next. Styrene eye markers were trimmed out and glued to the helmet base as markers for where the sculpted elements needed to be positioned.
I don't recommend the white apoxie sculpt to anyone looking to have clean, crisp details. I happened to purchase the wrong kind when shopping for sculpting supplies and my distributor is an hour from my house, so I was kind of stuck with it. The white apoxie has a more "fluffy" quality to it and won't take an edge or very sharp detail well. In my opinion, the "natural" color is much better. After a lot of shaping with clay tools and a bunch of water, the faceplate was textured and left to cure overnight.