See the Master MC achievement for a great explanation of how this works.
Basically, you need to create a custom arena with a lot of stuff in it and then drive around for a bit to "qualify" it. Once you've done this you can upload it to the leaderboard.
The leaderboard is practically empty so just fill the first arena with a variety of objects until the bar on the right is full and you can't add any more. You get points for the stuff in the arena, not for how well you drive while qualifying it.