Combine 2 parts mezcal to 1 part cubed ripe watermelon in a closed container, like a mason jar. (For a small batch, I used 1 cup of mezcal to 1/2 cup of watermelon. You can make a large batch with one 750ml bottle of mezcal to about 1/2 a large watermelon).
Let the mixture infuse for 24-48 hours (or longer for even more flavor!) Give the jar a gentle shake periodically.
After 1-2 days, use a fine mesh strainer to strain the liquid, pressing gently on the watermelon chunks to express as much liquid as possible.
Store any leftover infused mezcal in a closed container away from heat and sunlight.
For the cocktail:
Combine all ingredients in a cocktail shaker and fill the shaker 3/4 with ice.
Shake until chilled, then strain into a coupe glass.