This is achieved on Mission 5 (Research). Right at the beginning of the level, you will need to grab a keycard which is in a hut located at the top of the large hill. This is easiest pulled off by hugging the left wall as close as possible and keep moving forward.

The hill will be cluttered with patrolling mercenaries so you should mark everyone with your binoculars every 20 or so seconds so you can see them on your minimap. If an enemy turns yellow (suspicious) you can still unlock the achievement, however if they turn orange (alerted and searching) or red (hostile) then you will need to restart the checkpoint.

It takes about 1 minute to go from the start of the level to the hut with the keys, while hugging the wall, you can walk stood up, but do not sprint if there are enemies nearby as they will hear you, but crouching is not necessary either or you will be moving too slow. You can walk most of it, and even sprint when non enemies are close by.

See video for further help: