Wash the cabbage. Remove the outer layers 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 using some elbow grease for about 15 minutes. 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 to remove as many air bubbles as you can.
Fill the jar up to 2 inches from the top.
Pour any cabbage liquid from the bowl into the jar, so the cabbage slices are completely submerged in the liquid.
Place an outer leaf you had set aside on top of the sliced cabbage to help keep everything under the liquid.
Place a small jelly jar on top to weigh down the cabbage, then cover the jar with a cloth and a rubber band. Alternatively, if you use a flip-top mason jar, you can take out the rubber seal from the top then snap the lid closed. 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 jelly jar 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 10 days. Ferment longer if it needed. When it's tangy enough to your liking, store in the refrigerator. It will keep for months.