2 ounces gin
5 ounces tonic water
Put your ice in a highball glass, and squeeze in some lime. Add the gin, I prefer Plymouth, but New Amsterdam was all I could find.
Gently pour in your tonic water -- you don't want to lose too much of the fizz. I like Fever-Tree's Premium Indian, but any good one will do.
Squeeze in some more lime (Gunsmith Joe recommends "double the amount of lime you think should be in there"), and sip on your front porch while glaring at the neighbors and wondering if they have a flag.