This is another collectable achievement, thankfully you only have to collect 15 masks out of a total of 55. For this all you have to do is buy maps (the costs range from 20K to 120K, so I would buy the cheaper maps) at the tradings posts (found at outposts or randomly by passing by trading Sherpa's). Once you have done this, you just need to go to where it is on the map, now you will come across a search radius. The easiest way to find the masks is to unlock the gadget training skill (you can get this from the start, it is a first tier ability). This will allow you to see where the masks are by looking in camera and looking around the search radius (through the camera). The box in the middle of the screen will go green and it will say "set waypoint", the waypoint will not stay up but you just need to walk toward it with your camera out until you are close enough to see it.
Some of the masks may be below or above you, if this happens just find a way up/down. Also the camera may prompt you to "set waypoint" at other unique surroundings such as other collectables, so you may see multiple different prompts for other items.
I hope this helps you :)
Also, thanks to "Brian Beer God", some of the masks will require you too shoot them as they are either too high or just unreachable, this is why the achievement says remove or Destroy 15 masks.