Early in the game, you will witness a cutscene of people being chased in the snow by the Trinity soldiers. They see Lara's campfire and go to investigate, but she's up in the trees.
You'll be prompted to do a melee kill from above with the
button, and you'll drop down and need to hit
again to perform a finishing move. Quickly move to the bushes nearby — the game prompts you to use them as cover — and wait for the next enemy to approach. He'll be talking on his radio, and if you stay still, he will move up towards the bushes and the game will again prompt you to hit
. This will perform a stealth kill and net you the achievement. Very straightforward, but here's a video just in case.