Flip your car!

Here's how I did it:
Save up enough cash/tokens to get the Range Rover (77,000 cr).

Some people say you can do this with a stock Range Rover and no tuning. If you don't care or can't afford upgrades, skip ahead to the next section.
Optional
Buy the Race Suspension (adjustable)
Buy the Race Spoiler/wing (adjustable)
Buy as many engine upgrades as you can afford
Go to the main menu, Go to Tune/Upgrade
Press X to go to your Tune Car
Go to your Springs, Make all four bars max (Stiff and highest ride height)
Go to Aero and make your rear downforce 0/minimum
exit/apply/save/etc
OR
press "Select" (button with two squares) at the tuning screen, go to Load Tuning Setup, and search for my "flip range rover" tune. Also go download all my paint jobs and screenshots.


From main menu, go to Free Play.
Choose Sebring full circuit
Game Options, change max driveatars to 0
For driving assists, turn off ABS and set steering to simulation.
Start race.

Immediately turn around and go backwards. after one turn left you'll hit a huge straightaway. Floor it. Once you're near the end of the straight, hit your e-brake (press A) and turn hard to start sliding sideways. Hit the gas and turn your wheel so the tires are pointing "forward" (same direction that you're sliding). Try to hit the grass if you can.
This SHOULD get you flipping, but you might not get the achievement. Keep trying and you'll eventually turn your giant metal cube into a beautiful acrobat.