Cut the napa cabbage into quarters, remove the core, and chop into 2-inch pieces. Place in a large bowl.
Sprinkle with kosher salt and massage into the leaves. Add enough water to come halfway up the cabbage.
Press down on the cabbage to submerge into the salty water as much as possible. Let sit for 2 hours, turning every 30 minutes.
Meanwhile, make the kimchi paste by combining garlic, ginger, gochugaru, rice wine vinegar, sugar, and enough water to create a paste the texture of thick jam.
Test if your cabbage is ready to use by rolling a thick part of the leaf between your fingers. If it bends easily without snapping, it's ready. Otherwise, turn the cabbage again and let sit for another 30 minutes.
When ready, rinse the cabbage thoroughly to remove the salt. Drain well and return to the bowl. Add green onions.
Wearing gloves, massage the kimchi paste into the cabbage and onions until evenly coated.
Pack tightly into jars, pressing down to remove air pockets. Pour in any remaining liquid, adding water if necessary to keep the cabbage submerged.
For fresh kimchi, let sit for 2 hours and enjoy as a salad. For fermentation, leave the jars at room temperature for 1 to 5 days, tasting and then pressing the kimchi back under the liquid daily. Transfer to the refrigerator when the flavor is right for you.