First, you have to choose La Serennissima Repubblica di Venezia, AKA Venice.

Venice's ruler, Doge Francesco Foscarini, is a 4/4/4 ruler, so to make him a 5/5/5 ruler, you simply have to wait until his term is over (11 November 1448), and you have to have him reelected.

If Francesco Foscarini is not dead, than an event on 11 November 1448 will give you the option of 4 candidates to become doge, one of them being Foscarini. Simly click on "Keep Francesco Foscarini" and he will gain +1 Administrative skill, +1 Diplomatic Skill, and +1 Military skill.

Congratulations, you now have a 5/5/5 ruler and an achievement.