Go into Arcade Mode and choose Single Race, any difficulty. Choose the third track along: Trial Mountain Circuit. Any fast car will do, but i chose the Zonda R '09.
The rolling start will take you right past the bank you will be using to flip your car, so when the race starts: brake. Let the other cars go past and turn around (if you haven't already been spun around) to race the wrong direction.
Aim for the corner on your left, with the high bank. Hitting it correctly should flip the car at least once, but as long as you manage to get it on it's side, or roof, the trophy will pop.