My first time making natto and it worked perfectly, I'm so happy.
Let me tell you my easiest ever method:
The fermenter equipment is crucial, and in UK money it only cost new £25 ... nothing else. Here it is: https://www.ebay.co.uk/itm/263650353062 .... 'The Lakeland Multi Yogurt and Soft Cheese Maker'.
What is amazing about this unit is you can (a) set the hour times and have an hour countdown, and crucially (b) the temperature range to set the unit is broad at is 25 to 65 centigrade !!! That is a wide enough range for most legume and other ferments.
I set mine at 39C and it was ready in 24 hours exactly. I did not use the sieve insert but just put in a layer of cooked oganic soy beans with powder culture mixed in (2 small sachets, one in 80C hot water, other spread as powder over the beans and stirred in), and that's all, nothing else.
Natto is stringy goey and delicious, as it is, with no condiments, very mild flavour, no smells at all. Complete easy to assimilate protein (I am a vegan so appreciate a full protein source).
Next, want to repeat the process. If that goes as well, want to try with mixed legumes.
Then next want to try Koji ... rice first, then soy beans. Going to order the specific cultures (protease focus for soy, amylase focus for rice).
Then will give tempeh a try, have had repeat failures with tempeh.