This also helps unlock "Home Field Advantage":
Go to "My Home" & "My Profile" to see what your Signature Car & Favorite Location is.
Select "Solo Quick Race Weekend", select your favorite track location, select your signature car, set the race to 1 lap, 1 opponent, Same Class, Opponent Skill = Beginner, Grid Position = Player First.
Blue n Gold Eel says: ". . . made sure I was first on the grid, once it loads press A and once the game shows you on the grid with your usual viewpoint, press start and quit out, now it should register as a win. Do this 20 times, but on the 21st time you will need to win the race for the achievement to pop. Just do the same for the race wins once you get to 50 you will need to win the 51st race for the achievement to pop."
I did that but as soon as the game showed me in my car on the grid with me waiting for the red & green starting lights, I pressed "Start", then I selected "Skip To End Of Session", then "Restart", then "Yes", then "A", then "Start", & repeat that cycle for 50 times. It doesn't take very long.
Then win 1 race by driving the car yourself for the entire race (1 lap). Then "Return To Race Central". Enjoy!