Pretty simple, try to get all your cars in stock. This means that if you want to buy a car and modify it you must buy two, keep one stock for the challenge and tune the other.
So right now I have
2 500’s (new)
2 500’s (old)
3 4C (one tuned to 900, one with the launch pack body kit and one stock)
1 33 Stradale (stock of course)
and so on. I am up to the LP610-4, can you beat me??? (I decided to post this while some people still had monthly rewards in the bank to help)