Step 1Squeeze the lemons and pour the juice into a quart jar (you'll need 1 cup). Add the sugar and fill the jar with 3 cups of water. Cover tightly and shake until the sugar dissolves. Taste and add more lemon or sugar as needed.
Step 2To serve, pour 1 cup into a blender. Add 5 mint leaves and a handful of ice and process until slushy. Pour into a glass, garnish with a lemon slice or mint leaf and serve immediately, with a straw. Repeat with the remaining ingredients.