Peel the dark green skin off the watermelon. I recommend doing this after cutting the watermelon into quarters because it’s easier to hold onto pieces of watermelon about that size. Slice off all the pink flesh from the watermelon, leaving only the white interior of the rind. Slice the rind into 2-inch-long (5 cm) strips that are about ½-inch (1 cm) wide.

Pack the watermelon rind into a 1-quart (1 L) jar, leaving as little space as possible between the pieces. Sprinkle on the red pepper flakes and add the garlic. Pour over the kimchi brine.

Use a weight to keep the pieces of rind submerged in the brine. Leave the jar to ferment at room temperature for 3–5 days. Store in the fridge and use within 2 months.