Add all of the ingredients, except for the salt and pepper to the crock-pot. Cook on high for about 9 hours. Adjust salt and pepper after about 8 hours. After the 9 hours the beans should be tender and a bunch of them will have popped open. If they are not quite soft, let them go for a while, checking every half hour for done-ness.
Use an immersion blender or potato masher to smash the beans to desired creaminess. I like mine a bit chunky. Add more water as needed! Adjust salt and pepper to taste.
Traditionally, frijoles refritos were placed in a cast pan with lard and fried (hence the term refried) and mashed by hand, adding lard as you go to make a creamy paste. But I find this step unnecessary.
Serve with shredded Mexican cheese (optional).
Note This makes a huge batch (about 4 quarts) but the quantities can be reduced or the leftovers can be frozen up to 2 months.