Find your favorite rocks glass or any kind of small tumbler. Wet the rim of your glass with water (or lime juice) and then salt the rim by dipping it into a small plate of salt sprinkled with freshly ground black pepper and chili powder. Fill the glass with ice and set aside.
Place cilantro sprigs and lime juice in a cocktail shaker. Muddle the cilantro to release the oils then add watermelon juice, tequila, Lillet Blanc, agave nectar, and a dash of salt. Add a scoop of ice, then shake the cocktail vigorously for at least a minute. Strain into your prepared glass and garnish with a sprig of cilantro.
You can add a dash of salt to balance out the cocktail and spice up the salted rim with a little more chili powder and black pepper.