Make a saturated solution of salt in water, this is your brine. Add so much salt, not everything can dissolve
Buy fresh fish. You could use frozen fish, but fresh is preferred
Rinse the fish and let the water drip off
Weigh the salt and the fish and mix in a ration of 3 to 1
Pack fish and salt in a jar
Add brine. Ensure there is brine floating on top
Place a weight on top
Cover the pot to prevent most of the evaporation of the water
Check once in a while. After 180 days you can use
Strain the pot. Discard the fish or what is left. Use the liquid. if need be, filter over a coarse cloth.
The fish sauce can be stored for years at ambient temperature