Lots of people like using their slow cookers this time of the year, since it gives them more time to spend with family. I get lots of questions about cooking with beans and lentils, but there isn't much of a difference when using a crock pot than cooking them in the oven or on stove.
WATER is the biggest difference. Particularly with lentils, you need about 4 times the amount of water as you have lentils. For example, if you're cooking 2 cups of lentils then you'll need at least 8 cups of water. More if you add rice or beans in the same pot.
When using dried beans, whether black or pinto or whatever you choose, you will still need to soak them. You can do this in the crock pot overnight, just don't turn it on & remember to strain the water out before making your recipe. If only cooking beans (ie., no rice in pot but can have veggies) you should have about 2 inches of water or broth above the beans.
You can cook unsoaked beans in the crock pot, but the cooking time will depend on how fresh the they are, so you may end up over cooking the other things in the recipe if you don't presoak the beans first.
You can still cook veggies in with your beans or lentils, but anything that absorbs water (like the beans or lentils do) will need more liquid to be added. You might want to check on it every few hours, just to make sure there is enough liquid in the pot.
Other than soaking the beans, you don't need to precook them,except for kidney beans. They must be precooked to ensure they are safe! As little as 4 undercooked kidney beans can cause severe symptoms - and undercooked is more toxic than raw! You can precook them on the stovetop, just follow the directions on the bag to make sure they're cooked fully.
So, remember just make sure you have enough liquid on hand in case you need to add more and enjoy the extra time a slow cooker gives you.