There is a total of 36 airport you need to land at for this achievement.
For this achievement, chose your favorite plane to land, then click free flight. Zoom in on the map, right click near an airport, then left click on the plane. Click and drag the circle to it so it is facing in the same direction as the airports runway (little white line on each airport). Hit "M" and it will start you in the air with the runway lined up in front of you. Simply land and it will class as a landing. Once done, hit M again, and move onto the next airport. I recommend doing each island in turn and moving around the airports on each island in a clockwise direction so you don't miss one. lt may be helpful after each one to check your player profile and see whether:
Your total landings have gone up but your airports landed at have not (means you have already landed there)
Your Airports landed at has gone up
Neither total landings or your airports landed at have gone up (means it did not recognise it and you will have to repeat the landing)
It may also be helpful to make a list of which airports you have landed at to help you keep track.
Note: Bellows AFB (XBEL) is disused and therefore is not needed.