Would anyone happen to have step-by-step instructions to make Japanese fish sauce?
This is something to buy, not make.
Hatahata (or similar like mackerel, fresh anchovies etc) are placed in a barrel with salt (enough to cover the fish) for over a year. Then drain the sauce from the barrel. Longer periods of maturation result in a more complex flavour.
Use a clean barrel. Put in a layer of fish and cover with salt. The fish should look as if it has been snowing. Hatahata are quite small and can be used whole but if using larger fish like mackerel you may need to cut them up. Repeat with another layer of fish and salt and continue until the barrel is full. Put a weighted lid on top, cover and place the barrel somewhere cool, where it will not be subject to extremes of temperature. A pickle cellar or wine cellar are ideal. You want don't want the barrel to freeze, but you also don't want it to reach summer temperatures between 10 and 15 degrees should suffice. Leave for about 18 months. The fish will ferment and break down into a solid part (bones, scales) that falls to the bottom of the barrel, a white fatty layer on top, and a liquid sauce. Remove the fat and solids and filter the liquid through cheesecloth. Pasteurise the resulting liquid (bring to the boil, then allow to cool) and bottle in sterilised bottles or jars.
Nothing is required except the two ingredients, time, and careful control of temperature and cleanliness.