Add your chopped jalapeño and the juice of 1 lime into a cocktail shaker. Muddle the jalapeño for 1-2 minutes.
Add the watermelon to the cocktail shaker and muddle that down until it's mostly liquid.
Add 1½ ounces tequila (1 shot) to the cocktail shaker.
Add a little ice to the cocktail shaker and shake vigorously for 1-2 minutes.
Optional: Coat the rim of a lowball cocktail glass with Tajín seasoning.
Strain the cocktail into a lowball cocktail glass filled with ice.