4 to 5 oz grapefruit soda (I tried 3 oz of grapefruit juice and a small can of club soda)
Add tequila and lime and top with grapefruit soda.
Mix the ingredients around with a straw (or, as they do at La Capilla de Don Javier in the town of Tequila, with large knife), sprinkle a pinch of salt on top and garnish with a lime wedge or honestly nothing at all.