Soak beans in 3 cups of water overnight (8 hours or longer).

Remove skins. *This step is not absolutely necessary, but the end result is much smoother.

Cook beans in water in a large pot at high heat. Let boil for a few minutes, then drain.

In a clean pot with 3 cups of water, cook beans for 2 hours, taking care that water doesn't get below the top of the beans and they stay submerged. Or cook in a pressure cooker for 20 minutes. Check if beans are completely soft.

Take beans out of water with a skimmer, and retain the water. Process the beans in a food processor with 1/4 cup of the retained water (add a little more water if it's still too thick).

Put the bean mixture back into a large pot, add sugar, and cook until thickened. Cook about 5 minutes for softer Anko for scooping, and about 10 minutes for thicker Anko for shaping.