Very straightforward as the achievement suggests.
You have to liberate all 24 outposts in the game. Some outposts are not available until you progress past the invasion-bridge mission.
The outposts have alarms and some have heavy troopers, so come prepared with medical syringes and plenty of ammo.
I recommend going through all the radio towers first so you have access to the Buzzsaw MG42, because it's super OP and is great insurance if you can't stealth an outpost outright.
Please let me know if there's something you think I should add before giving a down vote.