First, unlock the Downhill Jam course in Career Mode.
Now, make sure you've used some of your money to upgrade your character's speed and air, at least a little.
Now make your way down the course, and keeping in the middle, until you shoot up a ramp onto a rock. Now make your way down the rock, onto the ramp and make sure to jump at the top of the ramp. If you make it far enough you will get 4 total gaps including the Ski Jump - 100 Gap. Make sure to press the A button if you get the "Big Drop" warning. Once you land it, you will unlock the achievement. If you screw up, restart your run and try again.
Refer to the video: