In a very large bowl, combine the sliced Napa cabbage and salt, tossing to combine and then massaging the cabbage for a couple minutes.
Fill the bowl with cold water, completely submerging the cabbage. Cover with plastic wrap and leave sitting at room temperature for 3 hours (it can also be left overnight). Drain the cabbage through a colander, then rinse it thoroughly. Allow the cabbage to dry. Wash the bowl to remove any excess salt.
In a small bowl, combine the Gochugaru, garlic, ginger, fish sauce, and sugar, stirring to form a paste. The more Gochugaru you add, the hotter the kimchi will be.
Return the cabbage to the large bowl. Add the chili paste, radish, and scallions, combining until evenly mixed. I recommend wearing kitchen-safe gloves and doing this step by hand.
Pack the kimchi into two 1-quart jars (or one 2-quart jar), pressing the mix down tightly. A brine should begin rising to the surface as you press the ingredients. If it doesn't, wait a few minutes and then continue packing the mixture into the jars (the vegetables will begin releasing their liquid). Leave 1 to 2 inches of space at the top of the jar.
Close the lid and then rest at room temperature for one week. Place a plate or bowl under the jar incase any liquid seeps out during the fermentation process. Once per day, push down on the kimchi with kitchen tongs or a spoon to help the brine coat the vegetables.
Once fermentation is complete, transfer the jar to the refrigerator. The flavors will continue to develop over time. While it can be enjoyed right away, it will taste better after another 1-2 weeks.
Store in the refrigerator for several months, until it becomes overly sour and/or loses its crunch.