First of all put your beans (I like great northern rather than navy beans) in a colander and wash them off. Yes, the packages say you no longer have to wash them. There's dust and sometimes little rocks in them as well as beans that don't look so good. I put a lb bag of beans into the slow cooker with water to cover about 1 inch overnight. The beans soak up most, if not all, the water. The next day, I add a can of beef broth, a chopped good sized mealy potato, a medium size chopped onion sauteed in olive oil or margarine (butter wants to scorch), 2 bay leaves, 1 T. salt, pepper to taste, celery if you like, a large chopped carrott, left over chopped ham or a meaty ham bone, smoked pork hock or whatever you have on hand, 1 small can of tomato sauce (rinse out the can with a little water and most important 1 teaspoon of sugar. There should be enough liquid to just barely cover. Cook on high until the beans are tender. Watch the liquid (you can add water) so doesn't cook too dry for your taste. Bean soup is the kind of dish that doesn't require amounts to be absolutely exact. It's kind of try it, taste it, adjust it.
A chemist who is also a serious cook wrote an article that explained that the sugar added to the beans during cooking, especially if they tend to be on the old side, allows the beans to cook tender without falling to pieces. I tried it many years ago and by golly it really works. The sugar doesn't sweeten the pot of beans.
Any leftovers, I freeze for another day. We prefer our beans not too soupy and have corn bread slathered with butter and honey and cole slaw as a side dish. There's enough starch in the beans, so fruit salad is light for dessert and finishes off a perfect blustery day meal.