To make grilled watermelon syrup:Cut watermelon into slices and grill until both sides of the watermelon have grill marks.
After letting cool, blend with simple syrup (1:1) and then fine strain.
Add all ingredients to shaker with ice.
Shake and fine strain over ice into Tajin-salted rocks glass. (3:1 Tajin to kosher salt)
Garnish with lime wheel.
Organise your soirée
Put the quantity of ingredients you have, to know how much other ingredients you need and how many cocktails you can make.