This is a story related achievement and will unlock once you complete the "Bury the Hatchet" mission. You start it with Michael when you walk into the lounge of his house.
The achievement itself will unlock just as you land on the airstrip with Trevor. Simple! (However, watch out for exploding planes and gun-crazed ambushes...)
Here's a video walkthrough of the mission with a gold medal in case you want it. Credit goes to GTASeriesVideos.