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From this point in the natto-making process, it's crucial that all of the containers and utensils you use are sterilized or mold and bacteria may develop. Boil a pot in water on high heat for 10 minutes to sterilize it. Dry it with a clean towel, and then add the beans to it. Boil any spoons or other utensils that you'll be using to mix as well. To make the natto, you'll need nattomoto powder, which contains natto spores. Use the special spoon that comes with the powder to add one spoonful of the spores to a small sterilized bowl. Add 2 teaspoons (10 ml) of boiled water to the powder, and stir the mixture with a sterilized spoon until the powder dissolves. Nattomoto powder is also known as natto spores or natto starter. You can purchase it from Japanese grocery stores and online retailers. Once the nattomoto powder has dissolved, cover the beans with the solution. Use a sterilized spoon to stir the beans and solution together so they're fully combined.

Summary:
Place the beans in a sterilized pot. Combine the nattomoto powder and water. Pour the natto spore solution over the beans.