Forza Horizon 2

After buying the required car from the HF Autoshow, you can easily get 20 drifts while doing laps of the diamond shaped area around the Autoshow. As long as you don't hit the walls while drifting, you should get this achievement after a few minutes.

Your first purchase of the 2015 Porsche Cayman GTS will be free so this wont cost you anything in game.

Any drift counts from a 0.1 second drift to a 60second drift (Bit crazy) but they only count as one so the best way is to initiate small drifts left then right. Drifts skills are performed by sliding the car into a turn for more than 10 meters, you can initiate a drift in a number of ways but the easiest method is to tap the e-brake with whilst turning.

Here's a video