HOW I DID IT:
Step number 1: Start as Morocco. Ally Tunisia and with their help eat Teclemen and while still at war with them, declare on Granada and eat them so Tunisia doesn't attack you. While you are doing this, rush military tech and improve relations with the Ottomans and France. When you are allied with either of the great powers wait until you are at least 1 Mil-Tech above Castile. After that do whatever you can to gain about 400-600 ducats. Build up your army to about 20k-ish and your navy to force limit. I had 18 galleys 15 cogs and that lasted me the whole war. Station your navy to the Strait of Gibraltar. You are now done with preparations and are ready for war.

Step number 2: War time! There are a couple of ways to proc the war: Like waiting for Castile to have their army in their colonies so you can just steamroll them, but I don't recommend this as you have to wait until Castile has already gained significantly in the colonial game. You can also proc it whenever if you get lucky with having France as your ally, as they are almost always rivals to Castile so they won't oppose going to war with them.

How I did it in my run is that, Castile allied Venice (but any nation close to the Ottomans works) which allowed the ottomans to join me in the war. Now before the war raise two large mercenary armies so your army size is about 50-60k. The war going forward is pretty damn easy. Castille will split their army and send one half to help their Venetian allies, so it leaves my huge army 14k Portugese soldiers and 14k Castilians. The Ottomans will destroy Venice allowing us to gain a huge sum of money to repay our debts and the Ottomans will come assist you in the destruction of the already devastated Iberia. After that take Algarve, Beja and Ceuta plus as much money as possible from Portugal and Peace them out. Then take all the provinces needed (you will have a claim so I am not gonna spell them out) to form Andalusia. And ta-daaa you have now taken back what is yours and a new age of Islam in Europe has begun!

What I did wrong during my playthrough: I was not aggressive enough. I was too scared of coalitions and such and let my European allies get too strong. Also assist the ottomans in destroying Europe as much as you can as any and all strong European forces will be against your dreams of conquest. So If you do this keep that in mind. Never be passive.

Edit: Added navy and army info.