Recipe from Delish.com
Remove from heat and stir in heavy cream and booze.
Do you cook the eggs?!
Not exactly: you temper them. Tempering eggs means you slowly add hot liquid to them to raise their temperature to avoid the risk of scrambling them. With eggnog, you warm up a mixture of milk, vanilla extract, cinnamon, and nutmeg and then slowly add that mixture to whisked egg yolks until it's completely combined. If you whisk in the yolks right to the saucepan, you'll likely end up with some egg curds (gross!).
How do you know when it's ready?
Once you temper the egg yolk-milk mixture, you return it to the stove and cook over medium heat until the mixture is thick enough to coat the back of a spoon. If you have a candy thermometer, it should reach 160º.