I rolled my car sideways a few times and didnt get this. Seems like you have to get airborne and do a flip in the air.
Here's how I did it:
Save up enough cash/tokens to get the Range Rover (77,000 cr).
EDIT : According to some of the comments, you can rent a Land Rover for free.
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.
[/optional]
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.
Xbox automatically saves a game clip for you when you pop this... After checking out mine, i had slid into the grass, when i was almost completely backwards, i hit the brakes. The Rover started rolling, hit the back bumper and launched into the clouds. Just uploaded my video to youtube. Posting it below.
Hope this helps!