I'll chip in from my experiences, mostly playing as Navarra (the flexibility of being able to land female dynasts and claimants is invaluable). Since Antso is more distantly related to the other Jimenas, this kind of start plays somewhat differently, as you have no claims on them, and they have no claims on you. (I imagine Aragon would be in a similar boat, also only being a cousin).
The first thing you should remember is that your fellow Christian Iberians are merely fodder to buy you time as you chew up the Muslims to the south. They will continuously do incredibly stupid things like fighting each other if not otherwise occupied. The first thing you need to do before even unpausing the game is to secure another, more reliable, ally. In the case of Navarra and Aragon, they start with a boatload of siblings, so you can actually secure more alliances with them. The three bigger Spanish kingdoms only have the king unmarried, so choose wisely.
I find that France and the HRE make good defensive allies, but are relatively poor at helping you take more land. The reason being as that those huge European powers need to muster all their troops, then either march them towards your lands, or march them to their coasts, get on a boat, and then get to your lands. By this time, the AI will have dogpiled you already if they were going to do anything (in the case of an offensive holy war). On the defence, it's not so bad because of the AI's reluctance to assault your holdings, instead preferring to wait for a siege, allowing the European doomstacks to arrive.
However, you surely need to take land from the Muslims if you hope to beat them back. For that, you need an offensive ally, and at the beginning of the game, I find that the best one is Denmark. Svend starts the game with literally a million kids, two of them being unmarried daughters. The best thing about Denmark is that all their counties are coastal, so he just raises his troops and immediately loads them on boats and comes to you. Also, I find that when his troops actually arrive, they will attach themselves to your primary stack, so they will essentially do your bidding. France and the HRE like to shuffle their stacks around independently, which doesn't accomplish much in the grand scheme of things. Winning holy wars (and indeed any war) is about ramping the warscore up to 100% to force peace on your terms, and the faster you can do this, the less time there is for Mauretania or someone else scary to intervene.
Another interesting choice in Matilda of Tuscany (and I only say interesting because I play Navarra and have siblings to spare). Since factions were introduced, I've noticed that Tuscany, and indeed most of the HRE that is NOT de jure territory, usually tables a successful ultimatum to the emperor. Sometime later they are reconquested, unless of course you can keep them busy fighting your wars instead of each other.
Once your alliances are set up, immediately declare a holy war for your nearest duchy and call all your allies. You basically want to storm the duchy and take it as fast as you can. This does several things. First, it obviously weakens the targetted Muslim emirate; you are essentially slashing their troop count in half for the next engagement, which will almost certainly be when you declare another holy war on one of their neighbours. It also immediately gets all your allies into the fight, fighting for your interests, ensuring they aren't expanding themselves, and making them easier to deal with when you decide it would be more convenient that their lands belong to you. Lastly, it will cause a truce. I know the truce appears to only apply to you, but I've never seen an AI attack me while a truce has been in effect after they lost land. It doesn't really matter that much, because they'll probably join the next holy war you declare on a neighbour, but at least you don't have to worry about them declaring a holy war on you.
The important thing to remember is to keep the pressure up against the Muslims while you can. If your allies are off fighting their own wars, don't declare a holy war right then, because they won't show up. Wait until they are at peace and then call them in. If you keep an eye on what your allies are doing and call them when they aren't busy, you can fairly easily roll over all of Muslim Iberia, even before holy orders.
That being said, do fear Mauretania. They can pull a lot of troops out of the three duchies closest to Gibraltar, and then cross over and ruin your day, which is why you need to make sure you can win a holy war quickly before you declare it. Don't bother crossing the strait until you have holy orders in hand, as you'll need the extra troops to smash their garrisons.
Once you control Mauretania, you can basically turn to cleaning up the other Christian Iberian kingdoms (or what's left of them), usually interspersed with holy warring your way to Tunis/Sicily. At this point, you no longer need to rely on allies and can attack wherever you can get claims. Usually for me this means continuing along the African coast until I say hi to the Fatimids (or whoever is running Egypt by now) by taking out coastal Egypt. You may also want to venture into Mali and take control before garrisons get too big, as it's a pain to haul troops through the desert by foot.
