Set up the ramp, choose your car, set the speed with the mouse button then sit back and watch the carnage.
Smash the star cars to own them.
Destroy enough cars to complete each level.
Good Luck!
If you get stuck trying to unlock a star car, check the video on THIS page:
http://www.makingfungames .com/portfolio/dac/
Oh, and 100% IS possible on every level!