If you have the Liars and Cheats downloadable content, you will see a total of 14 gang hideouts on your map in free roam. You do not need to complete all 14 of these hideouts to get this achievement. You only need to complete the following 7 hideouts (in one session):
You will know that you have completed a hideout when a stats screen appears after you've killed all gang members and/or completed all hideout objectives.
Going for this achievement can be a fun way to spend some free roam time with friends in a posse. Doing it in Friendly Free Roam will make it easier, because you won't have to deal with random other players occasionally killing you while you're trying to clear the hideouts. If you're rolling with a posse, it might be handy to start with Gaptooth Breach, as there is a stagecoach next to the final objective there, so you can load everyone into that and carpool to the other hideouts (just be careful going over narrow bridges like Frontera).