Quarter and remove the stalks from the cabbage, then chop it into 4cm pieces and place in a large bowl. Sprinkle over 4 tablespoons of sea salt and massage it in with your hands. Cover the cabbage with cold water and sit a plate on top to keep the veg submerged. Set aside for 2 hours. Tip the cabbage into a colander, rinse it thoroughly under cold water, then leave to drain for 30 minutes. Peel the garlic and ginger, then place in a food processor with the sugar and blitz to a smooth paste. Transfer it to a bowl and mix in the shrimp paste and gochugaru until combined. Matchstick the daikon and spring onions, then place in a large bowl. Squeeze out any excess water from the cabbage, then add it to the bowl, along with the gochugaru paste. Wearing disposable gloves, use your hands to mix it all together thoroughly. Spoon the kimchi into a large kilner jar and press it down so the juices rise, leaving a gap at the top, then close and seal the lid. Leave the kimchi to ferment at room temperature for 2 to 5 days (place the jar on a plate to catch any brine that might bubble over.) Check the kimchi every day, letting out some gas and pressing the vegetables down into the brine. When the kimchi tastes delicious, transfer the jar to the fridge for up to 3 months.