Partially cover and boil for 2-3 hours, regularly topping up the pan with boiling water. You should finish cooking them when the beans are tender and you have a thick soup in the pan. Allow them to cool and then drain them. If you do not have a food processor, you can mash them in the pan with a potato masher but this is hard work. Stir until they are thoroughly mixed and hot.
Frijoles keep well in the refrigerator and taste even better when reheated the next day.