Remove the rind from the watermelon slices using a sharp knife. Cut the flesh into rough chunks and remove any visible seeds. Place the watermelon pieces into a blender or use a hand juicer and blend on high speed for about 20–30 seconds until completely liquid. Pour the blended watermelon through a fine mesh strainer or a piece of muslin cloth held over a jug, pressing gently with a spoon to extract as much juice as possible. You should get roughly 200–250 ml of bright pink, clear watermelon juice. Discard the pulp. If you don't have a strainer, the unstrained version is equally delicious, just slightly thicker and more like a light smoothie than a juice.