Wash the beans and soak them in plenty of water overnight. The next day, drain the beans and put them in a large pot. Add enough water to cover them and boil over high heat. After boiling, drain the water and add cold water to the pot again. Repeat this process once more. Add water to the pot a third time and boil the beans again. After the water boils, reduce the heat and cook the beans until they are soft, about 1-1.5 hours. Check occasionally and add water if necessary. When the beans are soft, drain the excess water and mash them with a spatula or masher until they have a smooth consistency, add salt and mix.