Natto is traditionally an Acquired Taste--a stringy, stinky, almost moldy-looking ferment cultured with soybeans. I've heard that in Japan, it's eaten for breakfast with some raw egg yolks over rice.
Since soy is not something I'm super keen on eating much of, these days, I've tried natto-ifying various other beans, including black turtle and lentils. These produce natto that is milder than the soy version, but I'm hoping that the spores still manage to produce lots of Vit. K, etc.
I follow directions that are roughly similar to these: http://www.culturesforhealth.com/how-to-make-natto . Except that over time, I've started using about four times as much natto starter (four tiny scoops per 2 lbs. beans), I've found that sterilization is not necessary, and I just mix everything in the pot and dump it into containers to ferment in the dehydrator.
It comes out stinky and good, and this is one ferment that seems to have a calming effect on my child, rather than an excitable one.