At least 2 hours ahead, make simple syrup. In a small saucepan, add sugar and water. Bring to a boil over medium-high heat, stirring occasionally to melt the sugar. Remove from heat and let it cool to room temperature. Transfer to a container and keep in the fridge.
Put the watermelon cubes in a blender. Blend until smooth. Strain the liquid through a fine strainer to remove seeds and most of the pulps. Collect 1 cup of watermelon juice. Discard seeds and pulps.
In a medium jar, combine watermelon juice, lemon juice and cold water. Stir in 3 tablespoons simple syrup. Taste and adjust with more syrup if needed.
Put a few ice cubes into each serving glass. Pour the lemonade over. Serve and enjoy.
Notes
The sweetness of the recipe depends on how sweet the watermelon is. Make sure to do a taste test before adding the simple syrup.
Leftover syrup can be kept in the fridge for up to 2 weeks. It can used in iced coffee or cocktail.