In a small saucepan, bring sugar and ½ cup water to a boil, stirring until sugar dissolves. Remove from heat; let cool completely.
In the container of a blender, place half of cubed watermelon; process until smooth. Press juice through a fine-mesh sieve into a large bowl, discarding solids. Repeat procedure with remaining cubed watermelon, reserving 6 cups juice.
In a large pitcher, place watermelon wedges and lemon slices. Add sugar mixture, reserved watermelon juice, and lemon juice; stir gently. Serve over ice. Garnish with watermelon wedges and lemon slices, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/watermelon-lemonade/