After each mission (on Veteran difficulty) you are scored between one and three gold medals. For this achievement, you need all three gold medals on all six of the Navigation Missions. This can be done in single-player, local co-op, or online co-op. You can check your progress in the Achievements menu on the Main Menu, or on the mission select map.
For the Ring Race missions, you can get bonuses for finishing quickly, as well as for hitting less than three rings as you fly through them. In the Birds of War missions, you can get bonuses for finishing quickly, not dying, and hitting less than three mines. In the Follow the Leader missions, you get more points the closer you stay to the green portion of the leader's smoke trail, as well as points for executing the prompted aerobatic stunts, and bonuses at the end for executing all of the stunts and never bumping the leader.
The Ring Race and Follow the Leader missions are better done solo, since they're more about precision flying than eliminating enemies quickly. For the Birds of War missions, a partner might help you finish quickly, as long as they can fly as precisely as you. If you are having trouble getting all three gold medals on a Birds of War mission, try looking for a partner online.