Found this great guide on youtube, with commentary. Credit to Maka91Productions.
Text version:
Start the level 'Junkyeard Funk' in the Rustlands DLC event. To get here you must have finished training session 3 in the main game.
At the checkpoint just after the magnet with explosive barrels, stop and reverse slowly. Behind you is a red buttonn. Make sure your back wheel touches the button, then reverse further and fall down. Proceed into the hole that opens.
You'll now be in level 1 of the The Magnetic 10. Proceed to level 7 (they're all shown in the video). At the end of level seven, just before you enter the door the pipe behind you opens. reverse into it (make sure not to crash) and you'll be at level -1. Finish it to get the achievement. Once you finish the level you continue into level 8, so you could go on to finish level 10 for the 'The Magnetic 10' challenge.