Ranked > Custom > Yes > Basewar Capture the Flag > Create A New Match.
Go with 2 players, or with 4, or whatever even numbers you want. One person on the attacking team captures the flag, and then the person going after achievement just touches the other person's car to steal the first flag. After that, just keep your cars touched together, and the flags will automatically move back and forth. You can leave your control for 10 minutes, take a break, and come back and you will have the achievement. If you do not, start up the procedure again when the game ends, and before the second game ends (for me it took 2 seconds into the 2nd game) your achievement will unlock.