Place cut cabbage, carrots, onion, mustard seeds, caraway seeds, and salt in large bowl.
Using your hands, knead the ingredients together, squeezing firmly to help release liquid from the veg.
When the volume in the bowl appears to have reduced by half, pack the massaged veg into a wide mouth quart jar in layers, firmly pressing it down each time before adding more (the entire batch should fit into a single quart jar).
Press cabbage down firmly in the jar, so that liquid bubbles up over the surface of the jar. Loosely cap the jar (or top with an airlock, if you have one), position it on a small saucer or plate, and place it in a cool, dark spot.
Check every other day, removing any bloom and pressing cabbage down if it has floated above the liquid (be warned, it will be a bit stinky. That’s normal).
After a week, taste the sauerkraut. If you like the flavor, place the jar in the refrigerator. If you want something a bit stronger, let it continue to ferment until it pleases you.