Historically, it's somewhat "whatever" - Spiced ale or mulled wine (pretty broad range there) per one dictionary, but essentially you have license to make something that suits your idea of what it ought to be, and call it Wassail.
A random suggestion, starting from a cider base, since you posted here....
5 gallons cider, or 4 gallons cider and a gallon of AJC if you think it needs to be high-test.
A yeast you like the characteristics of - for me, an ale yeast, not too fancy, works fine.
Ferment that for a week, or all your delicate spice flavors may get blown off.
1/2 a clove (or none)
5 cinnamon sticks
A whole nutmeg, broken into chunks (or just one of the chunks)
Boil briefly to sanitize, cool, pour water and spices into primary. Give it two more weeks, move to secondary, hydrometer test, drink hydrometer sample, contemplate spicing more or not.
Two weeks later, bottle it.