To unlock this achievement all you need to do is press the back button and go to the map.
and all the fast travel locations will show up.
There are 3 fast travel types: Strongholds, Death Races which show up as a cup and outposts which show up as a balloon.
All of the outposts will show up as a solid white if it was visited already and a faded white which is noted as not visitied, same as races.
In order for the outpost to be activated as a fast travel location you must ascend in the balloon and the name of the region must pop-up, highlight all locations using your binoculars, not necessary but might as well do it since you'll need it, then close your binoculars.
After you do this on your last outpost achievement will pop.
Looting the outposts is not necessary for this achievement but is for other achievements.