Prepare a tall glass full of ice (preferably pint glass size) then add the Rum (about a shot and a half is probably good), next add the Ginger beer, add a wedge of lime to be squeezed into the drink.
Ginger beer is actually a mixer and not booze (staff? anyone?) I prefer Stewart's Ginger Beer, but any of the dark and spicy ginger beers will work, Bundaberg and AJ Stephans are other favorites.