Put the jar or vessel you'll ferment the mushrooms in on a scale, weighing in grams, then tare it to zero.
Add the mushrooms and enough water to cover them completely, then make a note of the total grams.
Multiply the total grams by anywhere from .03 to get the needed amount of salt, then add that much salt to the jar, mix well to dissolve, weigh the mushrooms down, (pictured was a bag filled with the same % of salt brine) seal the jar, and leave out at room temperature.
Burp the jar every few days to release carbon dioxide, or so until you're pleased with the flavor. Know that if any mushrooms poke out above the brine, even a little, they can quickly discolor, soften, and mold.
After fermentation, (5-10 days or so, I like to refrigerate mine after about 5) store the mushrooms in their brine in the fridge, making sure they're still completely covered with brine. The mushrooms will last as long as they're covered with brine, and will continue to ferment slowly.
You'll get decent flavor from these after 7-14 days of fermentation. After they are ripe to your taste, refrigerate them. You also need to watch these closely for mold, during the fermentation at room temperature, and after in the fridge. While mushrooms underneath the brine won't rot, the aroma of the mold can eventually permeate the whole jar if you forget about them.
You can vacuum-ferment the mushrooms by combining them with 3% of their weight in salt and vacuum sealing, leaving them at room temperature for the same time as fermented mushrooms in brine.