After completing the first few Jobs during your Online experience, Simeon will call you and tell you to bring a vehicle to the nearest Los Santos Customs shop in order to insure it. Steal any vehicle you'd like - although a fast sports car or muscle car is suggested - and head to the shop. There, you can insure the vehicle and add a select few mods which will all be free thanks to Simeon.
Upon exiting the shop, you will now be informed about the various Jobs and Races available around San Andreas. Open up the world map and scroll down the map legend until you get to Land Races. Set a waypoint on the nearest one and head over there. When you arrive you will notice a small blue circle at the Race start. Get out of your vehicle and walk inside the circle to start the race.
In the Race Settings Menu, change the "Laps" to ONE, "Vehicle Type" to whichever model your insured vehicle is (Sports, Muscle, Motorbike, SUV) and make sure "Custom Vehicles" is ON. Now invite a friend to the race and when selecting your vehicle, make sure it's your new custom ride you just customized at the shop. Once the race begins, communicate with your partner so they allow you to win the race. Once it is over, you can vote to replay the same race. Obtain four more victories and the achievement is yours.