For simple syrup, use a pairing knife to cut off the zest layer of a grapefruit. Try not to get too much of the white pith part. Just the brightly colored layer is what you want. Add that to a small pot with sugar and water and bring to a simmer. Stir to dissolve sugar, then remove from heat and let steep for 10-15 minutes.Simple syrup will keep in the fridge for two weeks.
TO make a margarita, run a lime around the edge of a glass and dip it in coarse salt. Then fill with ice. In a cocktail shaker add ice, tequila, grapefruit simple syrup, grapefruit juice (fresh squeezed is nice if you have the time), and a squeeze of lime. Shake well and strain into salted glass. Serve garnished with a zest section of grapefruit and a fresh lime wedge.