You need to have at least a little nitro to perform a Perfect Nitro Boost. Hit the right side your screen once to use nitro. A Blue circle with a nitro symbol inside in it will appear on your screen. Wait for the yellow line around the circle to touch the blue circle, before you hit the right side of your screen again (Should be around 1-2 seconds after you started to use nitro). You will be given the message: "Perfect Nitro", and your car will spark purple nitro. Immediately press the brakes, to stop using nitro, and repeat the whole process 19 more times. This can easily be done in a single race; Drift a lot, perform knockdowns and drive through the nitro pick-ups to get nitro. The achievement is however cumulative across all of your games, so don't worry if you don't get it in one game.