First, you will need some sort of sweetener such as sugar, honey, or even dried fruit like raisins. Captured between the tartness of the drink and sweetness of your sugar is the depth and character of the spices. Their scent wafts to your nose with the steam from the hot drink and you feel warm before even the first sip. There are many spices that you can choose for mulling and there really is no need to follow a recipe ~ cloves, cinnamon, nutmeg, ginger, mace, and allspice are all likely candidates. Often a bit of citrus in the form of a cloved orange, sliced oranges or lemons or their zest are added for brightness.
Go ahead and pull out the crockpot and mix up a batch of holiday cheer! If you feel like you absolutely need a recipe here is a version from Joy of Cooking which uses a mulled syrup rather than adding the spices directly into the wine:
Make a syrup by boiling for 5 minutes:
2 1/2 c. of sugar1 1/4 cups of water4 dozen whole cloves6 sticks cinnamon3 crushed NutmegsPeel of 3 lemons and 2 oranges
Strain the syrup and add to it:
4 cups hot lemon or lime juice
Heat well but do not boil. Add 4 bottles of red wine or Madeira, port or sherry. Serve very hot with slices of lemon or pineapple.