A rather easy achievement... You will unlock the car minigame by progressing through the story mode. The minigame has 3 different stages and you will need to get a cumulative score of 10,000 between them. You do NOT need to get it in one stage as that would be impossible. The faster you beat a stage, the better your score. It really isn't necessary to get the "flip" bonus as it usually slows you down anyhow. You can easily get 3500+ in the first two stages with a bit of trial and error. The last stage is hardest, so take it slow and make sure to land on your wheels so you don't lose health. If you do well enough on the first two stages a run around 2500 will still be enough.