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 will 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 (you can also use the back of a fork or spoon if one is not available). Add ice (preferably crushed) then add the rum and stir, and top off with the club soda. Garnish with a mint sprig.