As you're playing the game, you'll be traversing the open world map and conversing with friendlies in Hope County, and as you randomly travel Hope County and gain information, you'll be alerted to Cult Outposts. These Cult Outposts are in all three Seed regions, and some of them are required to continue on getting main story missions.
It does not matter what region you start in, before long you'll have to take on outposts to progress naturally in the game. It does not matter what outposts you take out when. Every outpost has at least two alarms that should be disabled by you to prevent further reinforcements from coming in. After that, it's just about mopping up and killing all the Peggy's inside that outpost. You'll know it's complete when it gives the location on your screen and says LIBERATED underneath.
In order to get this achievement, you need to either be playing solo, or BE THE HOST of a co-op campaign (which I have been this whole time) and liberate any five outposts and the achievement will be yours! Happy hunting!