Level: HardFor this one you will have to defeat several Muslim neighbors in order to form Portugal. Start as Duke Nuno of Portucale in 1066 who is a Vassal of one of the Spanish Kings.
Before you start, try allying France on day 1, but you have to divorce your wife before that. Go on a pilgrimage to get enough piety points. Use your French ally for the two back-to-back wars you'll need to take the south. Also create some Men-at-arms to help yourself. After you got the two Duchies in the south you have to start an independence faction against your Liege. Immediately press demands and call your ally again. After your flabbergasting victory you need 300 Gold to take the FORM PORTUGAL decision.
With the Fate of Iberia DLC you can Found Portugal without being independent as long as it's the Opportunity Phase (which the game begins in).
Credit to
Bayek von Kuwaq on Steam for his
guide. Information sourced from the
wiki.