Remove the damaged, outer leaves of the cabbage, wash the cabbage head and drain from water. Cut it into quarters, then finely grate (I recommend using a mandolin).
Transfer the shredded cabbage to a large bowl, add salt, and start massaging the cabbage with your hands to let it release as much water as possible.
After 10 min, the cabbage should have already released a lot of water, softened and shrunken in size a lot. If you are making one of the versions with spices, now you can add your spices and massage it for an additional 2-3 minutes. Transfer the sauerkraut mixture to clean, dry jars so that you leave at least 6-7 cm to the opening. Add the rest of the brine so that all cabbage is covered in it.
Take a clean plastic bag (I'm using a zip bag) prefilled with some tap water and carefully place it as a weight in a jar. All cabbage needs to be immersed in the brine. It's not necessary to close the jar with a lid.
Leave it in a darker place, at a temperature between 16-24 Celsius (60-75 Fahrenheit). Every once in a while check how the fermentation is making a progress. Sauerkraut is ready for consumption when you're happy with how it tastes.
Transfer the sauerkraut to the fridge to slow down the fermentation.