Cut the thick white part of the cabbage lengthwise in half. Then, slowly pull apart by hand to separate into two pieces. Do the same for each half to make quarters. Running the knife through all the way would unnecessarily cut off the cabbage leaves. Sprinkle salt into each cabbage leaves.
In a large bowl, dissolve the remaining salt in 6 cups of water. Submerge all the sliced napa cabbage into the salt water. Set aside for about 6 - 8 hours, rotating the bottom ones to the top every 2 - 3 hours.
Mix the rice powder with ¾ cup water (or optional dashima broth) and simmer over low heat, occasionally stirring, until it thickens to a thin paste, and cool.
In a food processor or blender, combine salted shrimp, Korean chili flakes, peeled and seeded apple, garlic, onion and ginger. Blend until it creates paste.
In a large mixing bowl, combine rice flour paste and red chili paste until well blended.
When the napa cabbage is ready, Spread kimchi paste on each napa cabbage leaf. (You only need to season one side of the leaf.) Repeat this step for the rest of the cabbage. Don’t pull the cabbage leaves off the stem, leave them attached so it holds together better.
Transfer the kimchi into a kimchi container or an airtight container (and put the lid on). Leave it out at room temperature for 2 days, then move it to the refrigerator. While you can start eating this once it’s chilled, you may want to wait 3-4 more days for it to develop more flavor.