So this is pretty easy, just win all four qualifiers and the semi final and the final.
What is not so straightforward is where this must be done: it will NOT count if you do this in My Team / Events / Career Rallycross, but instead must be done in Freeplay / FIA World Rallycross Championship.
You can set the difficulty to 1 and you just need to do this at one location (not for all races in the championship).
Update 2019-05-10: It is NOT sufficient to just win the qualifying races and the semi final, you also have to set the fastest time in each of the four qualifying races and also in the semi final.
Thanks to Ralph The Rogue for the extra info!