I did this with the Ferrari F12 Berlinetta that was given as the Tier 5 reward from
Forza Rewards. I made a setup that you can download if you want by searching for '7000 Drift Achievement' in the tuning setup section.
Get in your Ferrari and then head over to 'Tune/Upgrade' press X to tune the car and then goto setup manager with select. You can then press select again and then search for description '7000 Drift Achievement'. Load the setup, it's free.
Once the setups loaded go back to the main menu and choose Free Play. Select Indianapolis Motor Speedway (the forth one), I left the laps at 5. Before starting go to Assists
to turn off assists. This will help the car get sideways for longer.
I put braking line on for the clips so you can see where I started the drifts from.
Braking ABS OFF
Steering Normal
Traction & Stability Control OFF
Shifting MANUAL
Damage, Fuel & Tyre Wear OFF
Track is the oval, Brickyard Speedway.
Pause the game once it starts and
go to HUD and turn Drift HUD to ON.
I was doing about 170 coming into the first left after the long straights. Stay to the right on the straight and give it a bit of handbrake to start the drift as you turn left, then steer right into the drift to keep the back end from sliding out too far and keep the throttle full while you're sliding.
You don't have to do this all in one drift as you can see from the video I made, there were two smaller drifts after the big first one.
EDIT: Some people have said it notifies you of the Drifting away achievement multiple times before the achievement will be on your profile.
I did the rest of my perfect drifts on the test track accessed from the tuning menu by pressing Y as there are no cars to get in your way then. You can carry on round there as long as you need to get the Sidewinder achievement by doing 20 perfect drifts, anything above 5000.
The sidewinder achievement popped for me after I paused and finished the race/testing.