Forza Horizon 2 Presents Fast Furious

Air skills are performed once all 4 wheels are off the ground, perform 5 of these for the achievement.

The showcase race in the Jeep Wrangler is good for this.
Here's a video demonstrating each skill, Burnout, Wreckage, Drift, Speed, Air, Near Miss and Trading paint (In that order - Air @0:58)

All of the "skill" achievements are incredibly easy and you shouldn't even have to try for most of them. I unlocked this achievement during the race for the Jeep Wrangler, but you perform an "Air" skill every time you go into the air at all, and you will have plenty of opportunities during races and free roam to do so.

You'll know when you have performed one of the "skills" because it will display a little score near the top of the screen for it and say which skill it is ("Good [skill]!").