Raw cider vinegar is unheated and unpasteurized. Look for it at health-food stores.
Finely slice red cabbage, preferably with a mandoline. Place cabbage in a bowl, and add salt and vinegar. Using your hands to mix thoroughly, squeeze cabbage until it begins to wilt and get juicy. Let stand for 30 minutes. Adjust seasonings to taste. Add a little extra salt and vinegar if you are planning on serving the cabbage the following day, since the flavor fades as it marinates.
Cabbage can be refrigerated in a glass jar for 1 week.