Cut the Napa cabbage into quarters lengthwise, remove the core, and chop into bite-sized pieces. Rinse under cold water, then set aside.

In a large mixing bowl, dissolve sea salt in water to create a brine. Add cabbage pieces to the brine and ensure they are fully submerged. Let sit for 1-2 hours.

In a separate bowl, combine grated ginger, minced garlic, fish sauce, sugar, and Korean red pepper flakes. Mix well until you have a somewhat thick paste.

After 1-2 hours, rinse the cabbage under cold water to remove excess salt and drain well.

In a large mixing bowl, combine the drained cabbage, julienned carrots, and chopped green onions. Add in the kimchi spice paste, mixing thoroughly with your hands (wear gloves to avoid spiciness on skin).

Transfer the kimchi mixture into clean glass jars or airtight containers, pressing down to minimize air inside. Leave some space at the top as it will expand during fermentation.

Seal the jars and let them sit at room temperature for 1-2 days to ferment. Transfer to the refrigerator to slow down fermentation and develop flavors.