We recommend using a wide mouthed 1 or 2 quart mason jar to ferment the cabbage, as well as a small 4 oz canning jar to weigh the cabbage down and keep it below the brine.
Thoroughly wash the inside and outside of the jars, as well as the cabbage. Reserve one outer cabbage leaf, then quarter and core the cabbage, and cut into thin shreds.
Transfer the cabbage to a large mixing bowl, then sprinkle on salt, caraway seeds, and juniper berries. Toss to evenly coat and slightly knead the cabbage to begin breaking it down. The salt will extract liquid from the cabbage and begin forming a brine. Let mix stand 10-20 minutes.
Transfer the mixture to the mason jar, packing it into the bottom container either with your hands or a long tool. There should be liquid in the mixing bowl, go ahead and pour that in the jar too.
Place the outer cabbage leaf over the cabbage in the mason jar. This is to prevent cabbage floaties from making it past the brine—we want them to stay submerged. Then weigh the cabbage down with the smaller canning jar.
Cover the jar with a coffee filter or piece of cloth, and secure with a rubber band or twine.
Store the sauerkraut in a cool (between 65 F - 75 F) and dark place, checking on it every other day. The first day, I like to check on it more frequently to make sure everything stays submerged. Begin tasting it after about 3 days (longer if you make a bigger batch) to determine when the taste gets to your liking. If scum appears while it's fermenting, don't fret because that's normal. Just spoon it off and discard.
Once it tastes the way you like, transfer it to a storage container with a sealed lid, and store in the fridge.