There are six parts to this. Pork carnitas, crusty bread, marinated onions, two sauces, and limes. The limes are the easiest part and you'll want plenty on hand to squeeze over your sandwich while you're eating.
There are tons of carnitas recipes out there and you can use one that looks good to you, but traditional carnitas don't have lots of stuff added in like so many recipes do. Stick with Diana Kennedy's simple version here https://food52.com/recipes/13098-diana-kennedy-s-carnitas. Or put a hunk of pork in the crockpot with a good hit of salt and a bit of water and cook till it's falling apart. It's not going to be as rich as cooking it on the stove and rendering the fat into the meat, but it works. Shred or chop the meat and spread it on a baking dish and broil till it's a bit crispy, adding some of the cooking liquid as necessary to make sure it doesn't dry out at all.
The bread. This is the part where you're going to have to make sacrifices. Tapatios get a little dramatic about the birotes salados they use for tortas. Some US restaurants even fly in birotes from Guadalajara since they apparently can't be reproduced anywhere else. If someone in your town is making birotes, use those. If you can find decent crispy bolillos, they should work. Whatever you do, don't use the bolillos you can usually buy in the US because they're often more like teleras and completely wrong for tortas. It is absolutely essential that you use a bread that can stand up to being drowned in sauce. Personally, I get baguettes and cut them into smaller hunks depending on how hungry the eater is. They're not quite right but they work and are neither difficult nor very expensive since you can get three or four sandwiches out of one loaf.
The day before you want to eat these, thinly slice a large white onion and mix it with 3/4 tsp of marjoram or oregano or Italian seasoning, a couple tablespoons of vinegar, 1/4 cup of lime juice, and 1/4 tsp salt. These amounts are all flexible. Stick it in the fridge for at least 24 hours and they'll just keep getting better the longer they sit.
Most torta chains in Guadalajara serve two salsas. You can drown your sandwich in the spicy one, but usually people use a tomato-based sauce for drowning and an arbol-based sauce to spice it up. I recommend making both sauces so the eaters can choose how spicy their sandwich is. Both are really easy. For the spicy sauce, simmer about 15 arbol chiles (or whatever small, dried, red chile you have in your grocery store) and a small clove of garlic in a bit of water to soften them. Blend the softened chiles with a bit of the cooking liquid and strain. Add salt to taste, and you can add some vinegar if you like or herbs.
For the drowning sauce, simmer a kilo of cored tomatoes in a bit of water for 25 minutes, then blend with a tablespoon each of marjoram (or another herb) and salt, plus some sugar or tomato paste if the sauce is too thin or bland. You can also add vinegar and some people add more herbs or some onion. Everyone has their own recipe for the sauces so you can experiment a bit.
To assemble, cut open the bread and pull out of bit of the crumb so there's more room for the meat. Stuff it with plenty of meat (they add a lot of meat in Guadalajara). You can serve the sandwiches at this point with the sauces, onions, and limes on the table for people to add as they please. Or you can drown them yourself. These can be fun for a picnic if you bring the stuffed (but not drowned) sandwiches along with the sauces, limes and onions. But make sure to have a way to wash your hands because these are messy.
This makes a good ten sandwiches, if you used a big chunk of meat, with plenty of drowning sauce. Adjust amounts as needed, or freeze things for later like I do.