Cut a medium-sized watermelon into small cubes, removing the seeds and rind. You will need about 1 kg of watermelon cubes.
In a blender, add the watermelon cubes, juice of 1 lemon, and a handful of fresh mint leaves.
Blend the ingredients until smooth and well combined.
Pour the juice through a fine-mesh strainer into a large pitcher to remove any pulp or seeds.
Chill the watermelon juice in the refrigerator for at least 30 minutes before serving.
Serve the watermelon juice over ice and garnish with a sprig of mint or a slice of lemon, if desired.