Step 1
Place the cubed watermelon into a blender and blend until smooth. Strain the puree through a fine mesh sieve.
Step 2
In a saucepan, bring the sugar and 125 ml water to a boil over medium-high heat. Stir until the sugar dissolves, about 5 minutes. Remove from heat and stir in 750 ml of cold water and the lemon juice.
Step 3
Divide the ice into 12 glasses. Spoon 30-45 ml of the watermelon puree over the ice, then top with the prepared lemonade. Gently stir before serving.