As you travel the world you will gradually uncover more of the map. Hit Y, then select "Library" and finally "World Map" to see your progress. Look around for spots that are still beige and head there to uncover them. Very easy once you get Ba'ul. You will most likely be missing some small speck that you can't see with your naked eye, so once you do your initial sweep when you get Ba'ul, if you don't have the achievement just ignore it and finish playing the game. You ride him often going from city to city so you'll probably hit that last spot randomly somtime. If you don't get it before you finish the game for some reason, be sure to spend 300 in the Grade Shop to carry your map over.