Into a heavy pot. Bring to a boil then turn heat down to a simmer. Cover but leave vented about 1/2 inch. Stir periodically and add more water to keep beans submerged. After about 3 hours the beans will be very mushy. Let remaining water simmer away then. (Add water in small amounts, about 1/2 cup increments, slowly, down one side of the pot. You don't want to lose the simmer.)
This is what it looks like after it cools a little. It's not beautiful but it works great. Easier to digest than the intact beans you get from a can, and it creates a nice thick background for soups.
I store my bean paste in the fridge for a few days until I'm ready to put together a soup. I've slowly been transitioning my storage containers from plastic to glass. You can pick up these Pyrex bowls for a few dollars, very reasonable.
I use the same process as above for black beans, adzuki beans, kidney beans, pinto beans, etc. I also cook split peas and lentils this way, although I don't soak those overnight. And only red lentils form a paste.
Here is one soup using this cannellini bean paste I posted about recently: Cannellini Beans With Red Pepper And Rapini