Shred cabbage into thin strips and put it into a large bowl. Save the outside leaves to use in the last step.
Grate carrots and apple and add to the bowl. You can also add other vegetables if you wish. Add caraway seeds, if using.
Weigh the vegetables to calculate salt amount needed (2% salt). Sprinkle salt on the shredded cabbage.
Toss the cabbage to distribute the salt. Now comes the work, using your hands knead the cabbage like you would knead bread.
Mix the cabbage and press down with the palm of your hands and squeeze out the juice of the cabbage. Do this for a few minutes until the cabbage looks a little broken down and juicy. If it is not that juicy, let it sit for 15-60 minutes.
Spoon the cabbage mixture into a large glass jar or a fermentation crock and press down with a wooden spoon.
The vegetables need to be completely covered in liquid. In order to do that it needs to be weighed down so that there is a little pressure on it.
Place the outside cabbage leaves you reserved in step one on top of the shredded cabbage. This helps to keep it down under the liquid. If there is not enough liquid, you can add some water.
Place glass fermentation weights on top of the vegetables and push down so the liquid rises. If you don't have weights use a ziplock bag of water as a weight. Insert an empty plastic bag into the jar with the opening of the bag outside the top of the jar and fill with water. Close up the bag and press down gently to release any air and then close the jar.
Leave the jar alone to ferment for a minimum of 5 -21 days.
If you are using a mason jar without an airlock open the jar once a day to let out some air.
The longer you let the cabbage ferment the more different types of probiotics will form and it will taste stronger the longer it ferments. The time frame you could let it ferment is 5 to 30 days. Once you are done fermenting, removed the weights and whole cabbage leaves (discard) and spoon the sauerkraut into smaller jars and refrigerate.
