Ok, this achievement requires you earn a 1st place finish in all DiRT Tour Events, this includes ALL 4 Seasons as well as ALL 6 of the World Tour Events. the easiest way to achieve this is to put the difficulty in the lowest setting and race through all 4 seasons as well as the world tours that show up later on as you progress. You don't have to win every stage of the events, just make sure the overall finishing place is first (ie. A 3 stage event you can win 2 out of the 3 races and still come in first). This goes as well for the gymkhana events, the lower difficulty should lower the amount needed to get 1st place ( also just make sure your overall finshing place is 1st place). this is the fastest way to complete all races needed for this achievement. if you have any questions just ask i will answer with the best i can.
Note: Thanks to SockDust for confirming this, " IF you haven't bought the DLC, you DO NOT need it to unlock the achievement. IF you have bought the DLC, you DO need to get 1st place in the additional races to unlock the achievement."