This literally can be done in less than 10 seconds. Head to Multiplayer and select Private Lobby. Once there, select these race options:
Game type: DRAG (very imporant)
Track: 2-Car 1/4 Mile Drag
Max Players: 2 (you & AI)
Max Drivatars: 1
Drivatar difficulty: NEW RACER
Force Off TCS: OFF
Easiest Shifting Allowed: AUTOMATIC
Number of Heats: This doesnt really matter but I would put more than 1 considering it might take a few tries and you dont wanna wait forever between races.
Car Class: Doesnt matter
Drive Type: ANY
Now, at the starting countdown, roll your Right Stick around so that to view your vehicle from the side angle where the opposing car will be in the background - keep it that way throughout the race. One the race begins, do not floor it and let the opposing car take the slight advantage of a half car length (you'll easily be able to see it with the side view - you'll wanna see the other car's front tire past your front bumper; that's a safe enough distance). Once the race reaches 50% - it will be displayed in the lower left corner - floor it. you will win by half a second. Voila.
Should be done within 9 to 12 seconds depending on your car class. I used an R class.