Mix the yolks vigorously with the wire whisk for about a minute - they will turn much lighter yellow.
Whisk in 3/4 cup sugar and 1/2 teaspoon vanilla, whisking steadily. Grate* in a bit of nutmeg** (gently - you can always add more later, and fresh nutmeg can be powerful), then add 3 cups cream and 2 cups milk (and the booze, if you're using it) and whisk a bit more. Everything should be thoroughly incorporated.
* I grate nutmeg with a Microplane grater
- it's excellent.
** Use whole nutmeg, not the pre-ground. This is the primary flavoring in your eggnog, so it should be the good stuff. Whole nutmegs can be bought in the spice section at your local grocery, and they last far longer than ground nutmeg.