In a pan, add your custard and milk, whisking it gently on a low heat until full combined.
Give your cinnamon stick a little bash and add it into the custard mixture. Cover the pan with a lid and continue to warm through on the lowest heat for about 30 minutes. Stirring occasionally. You can substitute the cinnamon stick with ground cinnamon here, too. You could also get adventurous, adding in some other Christmas spices such as nutmeg.
Taste as you go. You may not like the flavour of cinnamon to be too strong, so once you're happy with the strength of the spices running through, strain your custard mix into a jug and leave to cool. You can speed this process up in the fridge.
Once cool, pour the eggnog over a little ice. Add a splash of booze if you like, and garnish with a grating of nutmeg, a sprinkle of cinnamon or even star anise.