To unlock this you need to have made it through to the Horizon Roster by moving through the seasons in the introduction over the first few hours of the game.
Once you have done this, open the Pause Menu and you should see a tile that has the current season displayed. Select this tile and you'll see the current seasonal events on the map.
You need to complete a Championship which will be a orange/pink race event icon on the map. When you drive up to it, you will see the conditions for earning a reward. This will usually be related to where you finish in the Championship and the difficulty you play on. BEFORE you enter the championship, go into your settings menu and change the difficulty of the Drivatars accordingly.
Complete all of the races and fulfill the criteria and the achievement will unlock during the rewards screen after the final race.
Additional Tip from Dellfis: You don't need to pass all difficulty critieria. You can just pass one. So if you get a reward for playing on Average, and you just want the achievement, just do that instead of going for all three rewards.