*Temper...
First dribble a small amount of hot milk in a steady stream into the eggs and sugar while quickly whisking the two together. This raises the temperature of the egg and sugar and gradually and cools the milk slightly.
Continue to dribble in the hot milk and whisk until you feel, with your hand, that the bowl's side or bottom has become warm or as close as possible to the liquid's temperature. Doing this is a good indicator of telling if the egg and sugar has been warmed up.
The eggs won't scramble or the milk won't curdle at this point, because they have been warmed gently. But, always keep the mixture moving - don't stop whisking.
You can make this alcohol free by omitting the bourbon.
Also put a shaker of nutmeg next to the bowl of eggnog for those who like to add it to thier drink