Complete Mission A-8 Bonus with a gold metal. Go to the cheats screen and turn on Carjacking chef and do any mission or do free roam mode (if you unlocked it) and press triangle to get of your car and find anothetr car to steal and press triangle to go inside another car.

On foot controls:

Triangle: get out of car (when stopped)/go in a car
Left Analog Stick: Move around