Making Douchi from scratch

Sandor Katz's Art of Fermentation has a recipe for douchi (link).

Soybeans (yellow or black) are soaked, cooked until soft, cooled, inoculated with Aspergillus spores or mixed mold cultures, and incubated around 80–90°F/27–32°C for about 72 hours, until the mold on the beans turns green, indicating sporulation. In contrast to the hamanatto method described above, in the douchi method the beansare not dried at this stage but rather washed, to remove the spores, which would impart bitter flavor. For the second-stage fermentation, the beans are fermented in brine, sometimes with sugar and/or hot chili paste, for four to six months, then dried.

That's a good instruction set on WikiHow that is linked above (and the comments in that article do note that they're working with the black soybean rather than the black turtle bean) but my mother, who ran a Chinese restaurant in Jacksonville Florida throughout the 1950's, taught me a much simpler method. It just requires more patience: Upon opening the last reserved jar of black beans, we would buy a pound or more of dry black soybeans,

1. rinse them well with cold or tepid water (NOT Hot) but do not dry them off
2. spread them out in a single layer, all touching (a cookie sheet with sides is helpful)
3. sprinkle a layer of salt (about a half tablespoon) on the still-wet beans
4. pour the salted beans into GLASS jars* and seal them tight.
5. set the jars aside in a dry place with steady temperature (uninsulated garages are not ideal)
6. The process is not uniform; getting 'older' stuff to touch 'newer' stuff will make the older stuff 'infect' the newer stuff within the jar. Therefore, gently shake or roll a jar once in a while to get different portions touching.
7. By the time we used up that last opened jar of beans (its not like we ate a tablespoon every day, more like a tablespoon every other week) about six months would have gone by and the new batch would have aged and fermented just from sitting around in that sealed container.

Contrary to the various instruction sets I've seen, we didn't discard any of the resulting product. Mom said it was like soy sauce: The older it gets, the stronger the flavor. She was confident it would keep forever because it was fermented, and we used it in dishes regularly enough that we never saw any go bad. Since I don't cook with douchi as frequently any more, my current habit is to leave dry beans in their original bag, then get a single 16oz jar started when I'm half-way through a ready-for-use jar. That's still letting the next batch ferment over about six months.

*the odor will NEVER come out of plastic containers

Are you wanting to ferment the beans or do the black bean sauce ?

Here's one link on how to ferment black beans.

https://www.wikihow.com/Ferment-Black-Beans

After that, there are many links on how to make black bean sauce, for example.

https://omnivorescookbook.com/pantry/fermented-black-beans