Rum, cola and lime over ice. The lime is not a garnish and not optional: it is the whole difference between a Cuba Libre and a plain rum and coke. Named in Havana around 1900 after the toast to a free Cuba. Two glasses, five minutes, nothing to cook.
The ratio here is strong: two ounces of rum to two ounces of cola per glass, which is one to one. Most bars pour a Cuba Libre at one part rum to two or three parts cola, so if you want it longer, top the glass up with more cola rather than adding rum. Use fresh lime juice, not bottled, which tastes cooked. Coke made with cane sugar, sold as Mexican Coke in the US, gives a cleaner finish than the corn syrup version, and diet cola does not work here at all.