If you plan to garnish your cocktail with a fancy fruit skewer, make that first. Trim your bamboo skewer so that it's about 1-inch taller than the cocktail glass you plan to use. Thread a cube of watermelon onto the skewer, a lime wedge, and a basil leaf. Repeat until the skewer is full. (It’s usually best to end on a watermelon cube for stability and presentation purposes, but don’t stress about it.) Briefly set aside.
Next, make the watermelon juice. Place your cubed watermelon in a blender and puree until smooth. Strain through a fine mesh strainer (yes, a pasta strainer will work if that’s all you have) into a glass or mason jar.
If you like a salt rim on your marg, pour some flaky sea salt onto a small plate. Moisten the rim of your cocktail glass and dip it in the salt.
Add the basil leaves and lime juice to a cocktail shaker and muddle gently. Add the watermelon juice, mezcal and a few ice cubes and shake vigorously to combine.
Strain the margarita into the rimmed cocktail glass and garnish with a decorative fruit skewer if you like.