Put the bottle of gin (it doesn't need to be full, just enough for a 2 ounce pour for all per drink) in your freezer 8 hours or more before you intend to serve your cocktail.
Put the highball filled with ice into the freezer at least an hour before you intend to serve your cocktail.
When it's gin and tonic time, remove your glass (or glasses) from the freezer along with the freezing cold gin.
Pour two, or a Bread Loaf three, ounces of gin into the highball, add 4 ounces of tonic or to the brim.
Squeeze the lime into it, adding the lime to the glass.
Cheers. (Recite your favorite poem.)