Red wine, minimum of two bottles, maximum depends on the size of your pan and number of guests. It doesn't need to be good wine to make good mulled wine, it just needs the red wine nature.
Apple juice.
Sugar.
1 tangerine, or an orange will do at a pinch.
cloves, in quantity.
A cinnamon stick.
Powdered nutmeg.
A very large saucepan, no, bigger than that, your very largest saucepan.
1) Two hours before you want to begin drinking the mulled wine, open the wine and pour it into the pan. Fill one of the now-empty bottles with apple juice for every two bottles, and add that to the pan. Add a tablespoon of sugar per bottle. Add the cinnamon stick or a half a teaspoon of cinnamon per bottle. Add half a teaspoon of nutmeg per bottle. Leave to sit.
2) Stick as many cloves as you possibly can into the tangerine. Add the cloved tangerine to the pan.
3) Put the pan to sit on the ring where it will be heated. Let it sit quietly. This is called "marrying the spices". I've often omitted this step due to lack of time, but it's well worth it. If you're going to go out and drink the wine when you come back, this is the point to leave it.
4) Three quarters of an hour before you want to start drinking, put the heat on really low under the pan and allow it to heat through slowly. You don't have to do anything. The sugar will dissolve, the spices will infuse, all shall be well and all manner of things shall be well. Don't turn up the heat impatiently, allow it to warm through very slowly. Taste for warmth from time to time, it should be warm. Don't turn the heat up, it really will warm through on its own, and you'll ruin it if you boil it.
5) Serve into mugs with a ladle. Leave on low heat and keep drinking until done.