Complete and win every event in Dirt Tour. If you're having trouble finishing an event, drop the difficulty. All in all, this is much more time consuming than it is hard.
There are a total of 100 events (91 regular, 5 World Tour, 3 XGames, 1 Colin McCrae). While most of the first level events are a single race, the later events up the number to 2-3 races each. In total there are about 250 races, so, as the title of the achievement states, this one will take some dedication.
Throwdowns DO NOT count towards this achievement.
For tips on not killing yourself out of monotony and other sanity-related gems, Supert3d's solution is a wonderful resource.