Rinse 2 cups soybeans and soak in plenty of water for 12 to 18 hours
Drain the soybeans and rinse again
Steam or boil the soybeans until very soft, about 4 to 6 hours if boiling or 1 to 2 hours if pressure cooking
Sterilize a bowl, spoon, and container for fermentation
Mix 1 to 2 teaspoons of natto starter or a small amount of store-bought natto with a little warm water
Combine the starter mixture with the warm cooked soybeans
Place the soybeans in a shallow sterilized container
Cover the container with a clean lid, plastic wrap with small holes, or breathable cover
Keep the beans warm at about 100 to 110°F for 18 to 24 hours
Let the beans ferment until they develop a sticky texture and strong aroma
Refrigerate the natto for 1 to 2 days before eating
Store the natto in the refrigerator and use within about 1 to 2 weeks