Ensure your mason jar is totally clean by sanitizing or washing it in hot soapy water.
Set aside one piece of cabbage that's slightly larger than the mouth of the mason jar (this is what we'll use to cover the rest of the cabbage).
Slice the rest of the cabbage thinly. Aim to make each piece similar-sized, so they all ferment at the same speed.
Place the sliced cabbage in the jar, pressing it down tightly to cram as much in as possible. Fill up to the neck of the jar.
Dissolve the sea salt in water to create a brine. Pour over the cabbage, right up to about 2cm from the mouth of the jar (you may have a little left over, that's normal as it depends how tightly you've packed the cabbage).
Fit the large piece of cabbage on top of the rest below the water line, tucking the corners down below the neck of the jar. This will keep the other pieces of cabbage underneath the water's surface while they ferment.
Place a sheet of paper towel on top of the jar, securing with string or an elastic band.
Leave to ferment on your countertop for 4 days, checking each day and topping up brine if any has evaporated. If a white scum appears on top of the cabbage at any point, simply scoop it off with a clean spoon.
After 4 days, sample it. If the cabbage is fermented to your taste, place a lid on the jar and store in the fridge (it will keep for several months). If not, leave on the countertop for a day or two longer, until desired taste is reached!