Cube the watermelon: Remove the rind and cut the watermelon into small cubes. You’ll need about 6 cups. Use chilled watermelon for the coldest drink.
Juice the lemons: Roll the lemons on the counter, then squeeze until you have ¾ to 1 cup fresh lemon juice. Strain out any seeds.
Blend the watermelon: Add the watermelon cubes to a blender and blend until smooth. Work in batches if needed.
Strain the juice: Pour the blended watermelon through a fine mesh strainer into a pitcher or bowl. Press gently to extract the juice.
Mix the lemonade: Stir together the watermelon juice, lemon juice, 4 cups cold water, and ½ to ¾ cup simple syrup. Taste and adjust with more lemon, syrup, or water.
Serve cold: Pour over ice and garnish with mint, lemon slices, or watermelon wedges. Stir before serving, as natural separation is normal.
