Make a simple syrup: Mix together 1/8 cup sugar and 1/8 cup water in a saucepan and bring to a simmer just until the sugar is completely dissolved. Remove from heat and add the mint leaves (keep some for garnish). Steep for 2-3 minutes, then remove mint leaves and discard.
Combine watermelon juice, lemon juice, salt, and sugar syrup in a jug and stir.
Serve with ice cubes, lemon slices and a few fresh mint leaves.