Slice up your seedless watermelon (little white seeds will blend up fine) into small chunks and blend. I use the NutriBullet so it liquifies the watermelon in a jif! But a regular blender should do the trick too, but you may want to strain it after blending to remove any pulp.
You should get close to 6 cups of watermelon juice from one small watermelon.
Mix equal parts watermelon juice and sparkling water. Add the juice of 3 lemons (1/2 lemon per serving).
Serve with a lemon slice and fresh mint sprig if you have it.