Throughout the game, there are several tan bricks with three spikes on top and a crystal orb in the middle. When you find one, fully press
to shout and pop the orb out. Pick it up with
and you'll be transported to a separate scene. The achievement will pop.
Though missable, the first one you can find is in the first 10-15 minutes of the game. Right after the deer you've been traveling with is captured, continue to follow the green diamond signaling your next objective. The first Silent helmet can be found at the bottom of a cavern in the area of the map with 10 crystal shards, 2 Silent helmets, and 12 hieroglyphs.
Note: Not every area has Silent helmets. To see if the area you're in has one, hold
to bring up the mini-map. If the middle number (in the upper-left hand corner) shows 0/1 (or more), that means there's a Silent helmet in your area.
Unlocking this will be the first of 12 you need for:
For a video guide, please consult this video created by Rare Ultra on YouTube. I take no credit for this video. The helmet I described is the first one found in this video.