Prep Glass: Rim your highball glass with salt: to do so, use a pomelo slice to slightly wet the rim of the glass. Then, delicately press the edge of the glass into the surface of a plate filled with salt. Set it aside.
Add Bitters (optional): If using, pour 2 dashes grapefruit bitters into the shaker.
Add Simple Syrup: Measure and pour ½ oz simple syrup into the shaker.
Add Lime Juice: Add 1 oz fresh lime juice into the shaker.
Add Mezcal: Finally, add 2 oz Mezcal to the mix.
Shake Well: Fill the shaker with ice, close it, and give it a good shake (10-15 seconds).
Add Ice: Fill your highball glass with ice cubes.
Strain: Using both the cocktail strainer and the fine strainer, strain the mixture into the highball glass.
Fill up: Fill up the glass with Squirt Grapefruit soda.
Garnish: Place a grapefruit slice into the glass for garnish (optional).