Quarter your Napa cabbage lengthwise, then remove the core. Slice each quarter into 2-inch strips.
Place the chopped Napa cabbage and sea salt into a large mixing bowl. Massage the sea salt into the Napa cabbage until it begins softening.
Once it starts softening, add just enough filtered water to cover the cabbage. Place something like a plate on top of the cabbage to weight it down. All of the cabbage needs to stay under the water, so make sure it's weighted down well.
Let the cabbage set for about 6 hours.
Pour the the cabbage into a strainer and run it under cold water for a few minutes. Leave the colander in the sink to drain for about 20 minutes so all of the water can drain out.
In a small mixing bowl, combine the ginger, garlic, sugar, fish sauce, 2 tablespoons of filtered water and Gochugaru pepper. Stir until a paste forms.
Squeeze any excess water from your cabbage place it in a large mixing bowl. Add the Daikon radish, green onions and your paste.
Carefully massage the paste into the vegetables everything is evenly coated. I highly recommend wearing food prep gloves as the pepper can sting a bit.
Once the veggies are evenly coated with the paste, transfer the mixture into a quart sized mason jar. You really want to pack it in there, so press down on it until the vegetables are covered by brine.
Leave about an inch of headspace (it will expand a lot as it starts to fermenand seal your mason jar with a lid and ring.
Place your jar on the counter on top of a towel (one you don't mind if it gets staineand let it sit for 2-6 days. You'll start to see bubbles in the mason jar and some of the brine may escape.
Every day, open the jar to release the pressure and taste your kimchi. The kimchi may expand significantly when you open the jar, so you'll want to press it back down under the brine before re-sealing your mason jar.
Once the kimchi tastes done to you, move the jar to the refrigerator for storage.
Enjoy your kimchi with your favorite Korean dishes - or all on its own!