Remove the outer leaves of the cabbage and set aside.
Shred cabbage in a food processor or by hand with a knife. I have done both ways and both are just fine.
Place the shredded cabbage in a large bowl and sprinkle with salt. Let it sit for about 5-10 minutes to allow the cabbage to sweat, this will make the next step easier.
Now it is time to make the brine! Massage, punch, and pound the cabbage with your fist until a decent amount of liquid forms in the bowls. The liquid is the brine and you will need enough to cover the cabbage when you transfer it to the mason jar.
Add small amounts of the cabbage at a time, start pressing it tightly into a glass mason jar. In between each added handful be sure to push down hard to see that the brine comes over top of the cabbage and to make sure their is no air pockets.
Continue to fill the mason har with cabbage. Leave about two inches of space at the top of the mason jar because the vegetables will expand. One large head of cabbage will make about a quart of sauerkraut.
Once all your cabbage into the jar you will need to use something to weigh the cabbage down so it stays below the brine. You can buy glass fermenting weights or you can make your own. You can use the outer leaves of the cabbage that you pulled off the cabbage in the beginning. Roll the cabbage leaves into log shapes and place onto of the cabbage. Or you can use a ziplock bag with a couple rocks in it. Be sure the bag is zipped so the rocks don't touchtone cabbage.
Place a loose lid over the sauerkraut and leave on the counter at room temperature to ferment.
You can leave it as long as 2 weeks or as little as 5 days. The longer it sits, the more sour it will taste. You can try it after 5 days and decide if you want to leave it out longer. Remove the weight, replace the lid, and store in the refrigerator. It can last up to 9 months.