Wash the cabbage. Remove the large outer leaf and set aside.
Quarter the cabbage and remove the core. Cut each quarter into thin slices down its length, about 1/8 inch thick.
Place the cabbage slices in a large bowl. Sprinkle with salt.
Mix, squeeze, and massage the cabbage for about 15 minutes, wearing gloves if needed. The cabbage slices will wilt and release liquid, and will look juicy at the end.
Place the cabbage slices into the mason jar by a couple handfuls, packing it down every so often with your fist or a spoon. Leave at least 2 inches of room from the top.
Pour any cabbage liquid from the bowl into the jar.
Place an outer leaf you had set aside flat on top of the sliced cabbage to help keep everything under the liquid.
Add a small jar inside the mason jar, pushing it down on top of the cabbage to weigh it down and submerge in liquid.
Cover the jar with a cheesecloth and seal the opening with a rubber band. This will ensure that the cabbage breathes without being sealed air-tight.
Store at room temperature away from sunlight. As the cabbage ferments, check to see that the cabbage stays under the liquid, and push down the jar inside if you see it rising. Also, the liquid may bubble and foam at the top. It may even rise and spill out. This is all completely normal.
Start tasting after 7 days. Ferment longer if it needed. When it's tangy enough to your liking, store in the refrigerator. It will keep for months.