Prepare a tall glass full of ice then add the Gin, next add the Creme de Cassis, next add the Tonic water, next add the Lime wedge. I sometimes add the cassis at the end and stir a little to take the excess fizz out of the tonic water if needed; it also creates flavour graduations throughout the drink. Don't overdo the cassis, though, unless you have a really sweet tooth.