In large mixing bowl, mix cabbage thoroughly with salt, juniper berries, and caraway seeds, using hands or tongs. If using your hands, make sure that they are very clean prior to mixing. Let stand for 10 minutes.
Pack cabbage mixture down into a large plastic food container. Top with a lid smaller than the opening of the container and place a glass jar filled with the quart of water on top of the lid. Place in cool area overnight (65 to 70 degrees F). In a day, the cabbage should have given up enough liquid to be completely submerged. The jar serves as a weight to keep the cabbage submerged and away from air.
Check cabbage every other day for approximately 2 weeks and skim the surface of scum, if necessary. Let stand for 4 weeks. Transfer to an airtight container and store in the refrigerator for up to 6 months.