In high power blender add 4 oz fresh grapefruit juice, 5 oz silver tequila, the froze grapefruit ice cubes and the 1 cup frozen watermelon cubes, 1/2 lime juiced, 4 tsp simple syrup & 1 tsp Grand Marnier.
Sugar the rims of your cocktail glasses (wet the rims and twist in granulated sugar of choice), place ice cubes in the cocktail glass, pour over the ice.