The season championship is a points based league across each of the 'elite races' in each episode.
you get 8pts for 1st, 7 for 2nd, 6 for 3rd etc.
For this achievement, you must finish in the top 3 after 12 episodes.
You can go back and re-do the events as often as you like, improving your result in an elite race will add the points to your total