Remove any damaged outer leaves from the cabbage. Cut the cabbage in half, remove the core, then thinly slice the cabbage into even ribbons.
Place the sliced cabbage in a clean bowl and sprinkle the sea salt over it. Massage the cabbage with your hands for 2-3 minutes until it softens and starts to release its natural juice.
Tightly pack the massaged cabbage into your jar. Pour filtered water over the cabbage until it is fully submerged, leaving about 1 inch of headspace at the top.