Build, rocks, highball, squeeze lime and drop into glass.
Dark 'n' Stormy® is a registered trademark of Goslings Export Limited.
People have enjoyed dark rum and ginger beer for hundreds of years, but in 1988, Gosling's Rum trademarked a name for the combination, and it caught on. Gosling's recommends only 1.5oz of rum and 4-5oz of ginger beer, presumably for sobriety / legal reasons. If made with another brand of rum, KC user inuakena points out that the drink should properly be called a Rum Mule to avoid infringing Gosling's trademark.