Winning races simple enough right ?
To earn this 20 Gamerscore you will need to win 5 online races with any of your beautiful custom cars that you've created. Keep in mind you must be online to earn this achievement. The less people in the race the easier it is for you to win the race. I did this achievement with a buddy of mine and was a lot easier to win.
Start up your on race, host your race on the prison racetrack because this track is very very short and can be completed in under a minute or two, then invite one of your friends to your game then make the game private turn off weapons so no one can mess with your achievement gaining. Then switch off winning races with whatever car you choose to use.
GOOD LUCK TO ALL !!