Place the mint leaves into a long mojito glass (often called a "collins" glass) and squeeze the juice from a cut lime 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 pictured below, though you can also use the back of a fork or spoon if one isn't available). Add ice (preferably crushed) then add the rum and stir, and top off with the club soda (you can also stir the club soda in as per your taste). Garnish with a mint sprig.