- Trim off the outer cabbage leaves (save them separately though, you'll need them later) and cut off the bottom of the cabbage head (the root). Then cut each cabbage in half lengthwise.
- Get a large basin and take two-thirds, or so, of the rock salt and mix it with the water. Soak the cabbages in this brine, being sure it gets into even the inner leaves, for 15-20 minutes. Drain, but do not rinse. Sprinkle the remaining salt evenly over the cabbage, again, being sure to get it between even the inner leaves. Let stand until wilted, usually 4-7 hours, though longer if the cabbages are especially large or thick. Be sure even the firmest parts of any remaining cabbage head have also wilted. It's a good idea to rotate the cabbages from the top to the bottom periodically during the wilting process.
- Chop the shrimp finely. Chop the garlic and ginger. Cut the green onions into strips about 1-1.5 inches (2.5-3cm) in length. Cut any radish or watercress into similar strips.
- If you are adding oysters remove them from their shells and wash thoroughly. Drain.
- When the cabbages are limp rinse them very well in cold water. You may need to rinse several times to remove all of the salt and brine. Allow to drain once the salt and brine have been removed.
- Cut the cabbages into squares of about 1-1.5 inches (2.5-4cm).
- Mix the red pepper powder with everything except the cabbage. Mix well. Taste, add salt or more red pepper powder as necessary.
- Now, starting from the inside and working out, place the mixture between the cabbage squares.
- Bundle the cabbage squares inside the outer cabbage leaves left over from step 1. Place inside a large earthenware pot to allow the kimchi to begin to ferment.
- After several hours taste the kimchi, pouring water (a cup or two) over it, into the pot, if too salty or spicy. If too bland, mix rock salt and/or red pepper powder with a cup or two of water and pour over the mixture into the pot.
- Once the taste is satisfactory and the kimchi has begun to ferment (it could take a day or two), place it in the refrigerator. Placing the kimchi into the refrigerator too early, before the mixture has begun to ferment, could harm its taste.