There are countless recipes for the Mojito, but apparently this version is for the one Hemingway himself enjoyed at the Mojito's place of birth: La Bodeguita del Medio in Havana, Cuba.
1 teaspoon powdered sugar
1 lime [or use Rose's Sweetened Lime Juice if you've run out of fresh limes]
4 mint leaves
1 sprig of mint
2oz Havana Club white rum
club soda
Place the mint leaves into a long mojito glass (known as a "collins" glass) and squeeze the lime juice over it. You'll want about two ounces of lime juice, so it may not require all of the juice from a single lime. Add the powdered sugar, then gently smash the mint into the lime juice and sugar with a muddler (a long wooden device, though you can also use the back of a spoon if one isn't available). Add ice (preferably crushed) then add the rum and stir, and top off with the club soda. Garnish with a mint sprig.
Havana Club, a nationalized rum in Cuba, is available all over the planet (USA excepted). If you find yourself in the USA, heading due North or due South or due East or due West for a while should eventually do the trick. Failing that, well, you'll have to accept an imitation.