This cheevo will be awarded for liberating 3 outposts.
The 1st will be part of the story and simply find another 2 from the map.
I would recommend using a sniper rifle to eliminate enemies from a distance.
If you manage to liberate an outpost with no alarms it will bag you another cheevo.
Also you will need to liberate 2 without being spotted to unlock a skill so bear that in mind when you approach an outpost.