I fly by the seat of the pants kind when it comes to cooking. This will probably infuriate those who need to know the precise quantities of things.
There are a number of different green chilies you can use: pasilla/anaheim, serrano, jalapeno, habanero. I like spicy so I almost always include jalapenos. I may include a regular green pepper or two if they're on sale. My secret touch is to slice all the peppers into 1" pieces (avoid touching the white pith running down the middle - that's where the capsaicin - alkaloid that we perceive as being spicy- is concentrated.) and blacken them in an iron skillet over high heat (no oil) in lieu of roasting them over an open flame. it's a good idea to have the hood fan going if you have one - the vapors coming up off those peppers can be brutal if you inhale them. The blackening of the peppers does seem to affect the flavor.
After blackening the peppers and placing them in the crockpot, use the same pan to brown the pork. I personally don't mind if the meat is a little fatty - it's not necessarily healthy, but it sure does taste better. If you choose to trim some fat off the meat, don't discard all of it. The size of the pieces should be roughly bitesized, but i tend to let the crockpot cook until the meat is tender enough to separate with a fork, I won't bother if I'm using country style ribs. This is optional, but I like to place the meat in a plastic bag, add 1/4 cup of flour and do the ol' shake & bake thing to lightly coat the pork with flour, as it helps to brown the meat, and that flour also binds with fat during the cooking time to thicken the cooking liquid. Brown the meat in batches (salt & pepper to taste) (too much meat in the pan lowers the pan temperature and inhibits browning) and place meat in crockpot. The meat will be in layers. I like to alternate the sprinkling of oregano and cumin between each layer.
If you have any leftover pork fat, place some in the pan over medium heat. Some of the fat will have been transformed to liquid, remove the solid fat. If you don't have the pork fat, use olive oil.
Saute 1-2 onions (cut into 1" chunks) & 5-6 peeled cloves of garlic in the pan until onions change color. Add to crockpot.
Add about 6 tomatillos sliced into about 1" chunks to the crockpot. (If you want to add green pepper, do it here) I like to add a dark beer and a couple of chicken bouillion cubes to the mix, YMMV. it makes the whole thing a little soupy. But as I prefer to eat this over rice, sauce works for me, and I will thicken the liquid by adding a slurry of 1/4 flour dissolved in 1/2 of COLD water, 1/2 hour before the pork is done. The flour binds with any fat that hasn't already bound with the flour that was browned with the meat forming a poor man's roux, and acts as a thickening agent.
Top with minced cilantro when ready - some recipes say put it in, but I personallly don't care for the result that way.
If the result is a bit too spicy for your taste, I recommend yogurt or sour cream - the milk proteins counteract the capsaicin alkaloids. I also would recommend sour cream if it seems too salty for whatever reason - sour cream with its relatively high acidity will reduce the apparent saltiness.
Chllies - call it 2 pasillas, 4 jalapenos 4 serranos
Pork - call it 4 lbs.
White Onion - 1 large, or 2 small
Garlic cloves (6)
Salt (to taste)
Pepper (to taste)
Cumin (approx 1 tsp)
Oregano (approx 1 tsp)
Tomatillos (6-8)
Optional:
Green pepper (1-2)
1 bottle dark beer
Chicken bouillion
Cilantro garnish