Slice the cabbage into 2-inch-wide strips. Salt the cabbage and put into a large bowl with salt. Massage the salt into the cabbage until soft then add enough water to cover the cabbage. Put a plate on top and weigh it down with something heavy (a can of beans works) so that cabbage stays submerged. Let stand for 1 to 2 hours.
Rinse and drain the cabbage: Rinse the cabbage under cold water 3 times and drain in a colander for 15 to 20 minutes.
Combine the garlic, ginger, sugar, and seafood flavor (or 3 tablespoons water) in a small bowl and mix to form a smooth paste. Mix in the gochugaru to taste – more equals spicier.
Squeeze any remaining water from the cabbage. Add it to a large bowl along with the radish, scallions, and seasoning paste. Mix everything together until completely coated.
Add the mixture into the jars packing it tightly until the brine rises to cover the vegetables Leave at least 1 inch of headspace. Seal the jar with the lid.
Let the jar stand at room temperature for 1 to 5 days to ferment. You may see bubbles inside the jar and brine may seep out of the lid; place a bowl or plate under the jar to help catch any overflow.
Check the kimchi once a day. Make sure the vegetables stay submerged under the brine. Check it daily for flavor and refrigerate when ready. You may eat it right away, but it's best after another week or two. Makes 1 quart