Cut a medium-sized watermelon in half and scoop out the flesh into a blender. Blend until liquefied, about 1-2 minutes. Strain to extract about 4 cups of watermelon juice.

Roll 4-5 large lemons on a countertop to break down the membranes. Cut in half and squeeze to extract about 1 cup of lemon juice, straining to remove seeds and pulp.

In a large pitcher, combine watermelon juice and lemon juice. Gradually mix in 4-6 cups of water or sparkling water to taste.

Incorporate sugar gradually; start with 1 cup, adjusting sweetness as needed.

Serve over ice and garnish with fresh mint leaves and lemon slices.