Mix the egg yolks with the sugar in a heated grog glass (or heat-compatible glass up to 200 ml, microwave) with a blender. Now stir the very hot rum into the sugar egg with a teaspoon (nice and slow, otherwise the egg will curdle). Well, depending on how much rum you like, fill up the rest of the glass with water or with more rum, until the glass is full.
For beginners I recommend 8 cl rum and 8 cl water. Increases are then still possible.