In large mixing bowl, mix cabbage thoroughly with 3 tablespoons salt, the juniper berries, and caraway seeds, using clean hands or tongs. Let stand for 10 minutes.
Combine the water and remaining pickling salt in a 1 gallon zip-top bag and seal tightly. Pack the cabbage mixture down into a large plastic food container or pickling crock. Top with a lid smaller than the opening of the container and set the zip-top bag of brine on top of this to act as a weight. Place in cool, 65 to 75ºF, area overnight. In 24 hours, the cabbage should have given up enough liquid to be completely submerged. If not, add enough brine from the plastic bag to cover cabbage.
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.