Start the game as a Catholic country that has one or less Cardinals. Which mean you can't start as France, which got two at the start, which disqualifies them for this achievement.
You need a province for each Cardinal so a larger country is to prefer. with that in mind I recommend Castile as a starting point, it has one cardinal from the beginning, you get a second after winning the war against Granada and converted their provinces to the Catholic faith. The third usually arrives as an event around the same time, which usually involved paying a small bribe in gold to convince one of France or Aragons Cardinals to join you.
During this whole process play nice with The Papal State and use your Papal influence to speed things up.