zsas, it's pretty easy, but time consuming, and like film developing, attention to detail and temps is everything.
The basic recipe is soak beans overnight, then either boil them for 5-6 hours, or pressure steam them for a much shorter time (< 1/2 hr), then sterilize everything from here on.
Drain the beans, you can leave some moisture depending on your local humidity level, mix in either some natto kinase starter per the instructions, or you can use a pre-packaged pack of natto, if you don't have any starter. Layer it about 1-2cm high max on trays, covered with sterilized foil, and maintain it at 30-40 deg. C for 18-36 hours or so depending on preferences.
stir it up, let it get to room temp, refrigerate a few days, then freeze what you won't eat in the next week.