Ok, this is simple enough but very time consuming. There are 13 different cars and each has 4 styles (you do not need to worry about the colour of the car). You must complete an activity with each car and style in full and you cannot quit out otherwise it wont count. My personal choice for this was the Dash activity as it only takes approx 35 seconds if you race the very first one in the events list. The frustrating part about this is the loading time as you have to quit to the main menu after each race to change your car or style of car.
PLEASE NOTE - Keep a very close eye on your Gamertag at the beginning and end of each race as you must be shown as in control at the start and finish of each event, the kinect camera has been known to lose the player at times and this can completely destroy your progress if you dont notice it, and in turn you will have to go through the whole process again until you find the one you may need. Be especially vigilant if you take a break as the camera will lose you and show as a guest.
So in short, 52 races all together, boring but worth big points for your true achievements score.