Self explanatory, and you will unlock this while attempting the other season-long Achievements. I recommend focusing on Career Mode since you will have to complete multiple seasons anyway in order to unlock the following:
One thing to note is that you do not need to take part in every Session; you can skip Free practice sessions if you wish.
You also don't need to finish every race, so feel free to Retire or Disqualify yourself if you like.
Finally, you can Simulate races if you wish, your Achievement progress won't be affected.