There's two things you're going to need first before you can attempt the achievement and they're, one other player or a spare controller and a user-created track. If you don't have a friend with you or another player to race against, you can find some [B][URL="http://www.xbox360achievements.org/forum/showthread.php?t=366429"]HERE[/URL][/B]. For the custom track, you can either use one you've personally built or simply download one from the hundreds the game has to offer.
Once you've got both of the above, head into the multiplayer menu and set up a private game or a local game if you're playing with a friend/spare controller. Now invite your other player into the game and race on the custom track for the achievement to unlock.