I Am a GPS

Per the overall roadmap, this is one of the achievements that is best done as early as possible in a career with no map DLC enabled - as any map expansions just increase amount of map geography that you have to cover.

Keep track of progress on the in-game map. The % discovered is shown in the top left corner.

After this, which is relatively easy, keep going for 100% discovery to unlock Pathfinder (but be warned, its a massive grind!)