From Greg Andrews of The Pickled Chef Co.
1 (2-pound) head Napa cabbage
1/4 cup sea salt or kosher salt
1 tablespoon grated garlic (about 5-6 cloves)
1 teaspoon grated ginger
1 teaspoon sugar
2-3 tablespoons seafood flavor
1-5 tablespoons Korean red pepper flakes (gochugaru)
8 ounces Korean radish or daikon, peeled and cut into matchsticks
4 scallions, trimmed and cut into 1-inch pieces
Cut the cabbage lengthwise into quarters and remove the cores. Cut each quarter crosswise into 2-inch-wide strips. Place the cabbage and salt in a large bowl. Using your hands (gloves optional), massage the salt into the cabbage until it starts to soften a bit, then add water to cover the cabbage. Put a plate on top and weigh it down with something heavy, like a jar or can of beans. Let stand for 1-2 hours. Rinse the cabbage under cold water 3 times and drain in a colander for 15-20 minutes. Rinse and dry the bowl you used for salting, and set it aside.
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, using 1 tablespoon for mild and up to 5 tablespoons for spicy. Gently squeeze any remaining water from the cabbage and return it to the bowl along with the radish, scallions, and seasoning paste. Using your hands, gently work the paste into the vegetables until they are thoroughly coated. The gloves are optional here but highly recommended to protect your hands from stings, stains, and smells!
Pack the kimchi into the jar, pressing down on it 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 1-5 days. 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, pressing down on the vegetables with a clean finger or spoon to keep them submerged under the brine. (This also releases gases produced during fermentation.) Taste a little at this point, too! When the kimchi tastes ripe enough for your liking, transfer the jar to the refrigerator. You may eat it right away, but it’s best after another week or two.