Place the cabbage in a large bowl and sprinkle with the salt. Massage into the cabbage until plenty of liquid is released and the cabbage softens, about 10 minutes. Toss with the beet and juniper berries.
Pack the sauerkraut into a sanitized container, pressing the cabbage beeath the liquid. If necessary, add a bit of water to cover the cabbage. Use a small jar filled with beans or another clean, nonporous, weighted item to keep the cabbage submerged beneath the liquid. Cover the jar with cheesecloth and secure it with a rubber band or twine.
Let the sauerkraut ferment at room temperature until it's bubbly and tangy to your liking, 3 to 10 days, then refrigerate.