I managed to unite the Iberian peninsula as Castille in my latest game, and I am by no means a great player. The way I went about it was fourfold - firstly unite the main Catholic kingdoms, then save up tons of money, ally with a powerful kingdom and, most importantly, pick your moments.
I did start with a ruler-designer character, but honestly this was more harm than help, as it means you lose the benefits of the intra-dynastic alliance the Jimenas all get. However you do keep your ruler's claims, which helps (I believe the kings of Castille, Leon and Galicia all have claims on each others' titles). In my case I turtled a bit until I had 1000 gold, faking claims on my vassals' counties in the meantime and waiting until I had a child old enough to marry off to a French prince/princess. Once you have said alliance and sufficient gold, wait until the other Catholic kingdoms are weak (preferably infighting, if not then when they are fighting the Muslims) and hit them hard with the aid of the French and as many mercenaries as you can support.
If successful, you'll now control all of Northwest Spain. Take some time to consolidate your position and put down any uppity rebels (I went for the full on tyrant route with my insane ruler, banishing anybody of count rank or above in order to get tons of cash, but that's optional). When you want to take the fight to the Muslims, just do the same thing as before - save up money for mercenaries, call in France, and try to wait until they're infighting if possible. If you want to speed the process up, declare holy war on as many of the Iberian powers as you can simultaneously - they'll all join each others' wars anyway.
The holy orders are helpful when they arrive, but I think by the time they did I hardly needed them. However, if you get a string of bad luck they can definitely help in an important war.
I hope that helps!
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