To unlock Snapping turtle in Ghost Recon Breakpoint, you'll need to use the game's new prone camo feature. When you're laying in natural ground, i.e. dirt, grass, mud, or snow and not flooring, road, or other manmade surfaces, you can press A to cover yourself in the soil below you and hide.
While doing this, your movement options are limited to aiming only within a certain range so that you don't move and reveal your location. You can still send up a drone, however, and you can even shoot from that position, provided your target is in that limited range.
If enemies approach you without flashlights and when they're not on alert, you'll be able to melee kill them by clicking the right stick as they get right up next to you. But if they catch you in their light or are already scanning the area for you, they'll spot you before you can get in for the kill.
If an emeny is stationary, like two early enemies are when the game begins, you can also slowly crouch behind one, go into prone camo, and net the kill then, maybe making it the first achievement you unlock in Breakpoint.