I'm proud of this.
Started by crushing the Castilians since I guaranteed Granada. They went bankrupt quickly from naval maintanence (game was started 3-29, lol) and dropped their military maintanence to 0 on both. I assaulted all their forts and killed their demoralized armies. A few wars later, I had pushed them to the background in Galicia and Alicante, surviving along with Portugal because of my good graces. (I'm so nice...) Proceeded to murder the Granadans who I had saved earlier, then when France inevitably balkanized I ate the southern minors.
Finally, I set about the quest of unifying Iberia. It was actually a rough nine or ten wars to get all the provinces, because Portugal and Castile were allied and Portugal had a navy twice the size of mine, I simply couldn't get all of it in a small amount of wars. Also, Portugal always gained alliance leader and called in Scotland, Holland, and Savoy. Savoy was huge, at least five times the size that it is in the picture. They would rampage through my french possessions and annihilate at least two armies a war. Somewhere around the seventh war I vassalized Castile (I couldn't reach the colonies and they only had one province in Europe) so I had to DoW portugal directly. This brought almost all the french minors down on me, because apparently they had decided it would be smart to guarantee a nation thrice the size of them??
Anyway, after the ninth war where I really focused on Portugal, I shattered them enough to make Savoy gain alliance leader in the tenth. I finished off taking Portugal's two Iberian provinces via occupying their South American capital from my newly acquired vassal's (Ragusa) South American provinces (I've only colonized a bit of Canada in this game, and a small bit of Africa as strategic landing points against potential enemies) I then turned my attention to the very large thorn of Savoy. I murdered them: took three provinces, cancelled half their alliances, and forced them to release two nations. Nice was actually mine but it ruined my borders so I sold it back.
Anyway, after DA-ing Castile a few years before this picture was taken, the unremarkable Ferran IV de Luna was proclaimed Crown of Castile, the King of Iberia.
Edit: I can actually become Spain, but Aragon's color is too amazing. Also, there should be an Iberia decision with a similarly good color.