Cut the cabbage lengthwise into quarters and remove the cores. Cut each quarter crosswise into 2-inch (5 cm) wide strips.
Place the cabbage and salt in a large bowl. Using your hands, massage the salt into the cabbage until it starts to soften, then add enough water to cover the cabbage. Put a plate on top and weigh it down with something heavy, such as a can of beans, or a mason jar filled with water. Let stand for 1 hour.
Rinse the cabbage under cold water and drain in a colander for about 15 minutes.
Meanwhile, combine the garlic, ginger, sugar, and fish sauce, if using, in a small bowl and mix to form a smooth paste. Mix in the gochugaru, using 1 - 2 Tablespoons (5 - 10 ml) for mild, or up to 5 Tablespoons (25 ml) for a real kick.
Combine the cabbage, daikon, and scallion with the paste. Mix everything together with your hands, or use a pair of tongs.(If you use your hands, gloves are highly recommended to protect your hands from chili stings, and smell.)
Pack the kimchi mixture, including the liquid that has been released, into a quart-size, wide-mouth mason jar, pressing down on it firmly with a wooden spoon or rolling pin until the vegetables are level with the shoulder and there is 1 inch (2.5 cm) of brine above the top of the vegetables.
Place Kraut Source onto the jar. Allow to ferment for 5 - 10 days in a cool spot, away from direct sunlight. Letting it ferment for up to 14 days is even better. Check every few days that there is water in the moat, and top off as needed (see Note).
Replace Kraut Source with the standard mason jar lid and ring. Transfer to the refrigerator.Note: As Napa cabbage releases a lot of water, keep an eye on your kimchi during the first 24 - 48 hours. There should be about 1 inch (2.5 cm) of liquid above the top of the vegetables, so pour off excess if it looks like it will over flow.*Gochugaru, or Korean red pepper flakes, is widely available in Asian grocery stores.
