It's an easy achievement.
You have to win a stage in any rally you want. Before actually starting the race make sure you go to the configurations menu and change any part you want*. I simply lowered the handbrake strength by 1 point to not affect the driving too much.
Save and exit, then race stage and win it. You can even do it on the lowest difficulty.
*If you stay in the configurations menu for 10 minutes you'll also unlock this easy achievement:
(you don't even have to do anything, just be in the menu)
Thanks to WolfheartSpell for help.