Pour kosher salt onto a plate. Use a grapefruit wedge to rub the outer rim of a highball glass and dip the rim in the salt, careful not to get any on the inside of the glass.
Add ice to the glass. Pour in tequila, lime juice and grapefruit soda and stir gently. Serve with a grapefruit wedge, if desired.
Notes
If you prefer to use fresh grapefruit instead of grapefruit soda, here's my go-to recipe. You'll need: