When you are able to go on a ramp, you will fly off the ramp at the end of the ramp. The time you are in the air will add up throughout the race & all races. I would recommend you to go on the slant ones to get this achievement
as doing barrel rolls counts as time in the air. You will be able to track your progress at the end of the race as well which is a big help so you know how close you are to finishing this achievement.