Make watermelon ice cubes by putting 24 pieces of cubed watermelon on a baking sheet lined with wax paper. Freeze for one hour, covered.
Take the rest of the watermelon plus the sugar and puree until smooth. Strain into a bowl. Put the tequila, basil and mint in a pitcher and muddle. Then stir in Cointreau and watermelon puree. Pour mixture into cocktail shaker with ice and mix for each cocktail. Put watermelon ice cubes in a glass and pour cocktail in. Garnish with herb sprigs and enjoy!