Another easy achievement here. The first 'challenge' that pops up is to complete a lap of the beginner course in 20 seconds - it only took me two tries. The key is to drift the last bend - if you run automatic, it's actually harder to control the drift. I believe that this is the default and only gear selection available for the challenge.
However, if you can, my suggestion is to do this on manual, and just before you turn into the third bend (after the slots), shift down to SECOND gear by tapping LB (default) twice. Turn in, then immediately tap RB (default) twice to go back to fourth gear and maintain speed. Then, just keep it off the walls and the grass, and get across the line. If on auto, you will need to temporarily slam the brake without letting off the gas and turn it, losing traction and drifting throught the bend.