In a Martini glass fill ice and soda water to chill. On the side in a drink shaker fill with ice, add the grapefruit vodka, grapefruit juice, and cranberry juice topped off with a spash of lemon-lime soda. Sugar rim optional after you dump the ice and soda water out. shake ingredients and strain into chilled glass. you may also serve with a portion of grapefruit slice. enjoy.