Grab a livery, grab a tuning setup, buy a car, and enter a Showdown at any Car Meet-10G

This can be done on four separate occasions and not necessarily in one go. Select ANY other car at the meet and you can choose to buy the livery (which will be then be downloaded to your Designs), Buy the Upgrades (if the other car has a tune which is available freely on the T10-servers) and Buy the Car (which basically takes you to the Autoshow to purchase that car). Not everyone has got a Tuning Setup on their car; ask a good friend to make any tune, share it as usual on Forza games, drive up to the meet so you can grab it. And the last one is Enter a Showdown which is basically pressing X and racing against (at least) one other player.