The history of mulled wine is a conflicting one. The more cynical might tell you that winemakers in the Middle Ages mulled their wines to cover up poor quality. While there are other reports that it was the supposed health benefits of the spices that drove its creation.
Variations exist around Europe, with German glühwein popular amongst skiers. Yet it was Sweden that kept the love of the drink going and specifically introduced the association with Christmas.
What wine to use? For a red: go hot climate, with some big fruity flavours. Don’t worry about tannins or residual sugar here, nor look for subtle flavours. That’ll be lost once you’ve heated it. For reds: something floral should work quite well.
The key thing is not to boil the wine else you’ll lose the alcohol. Keep it on really low and heat it slow. Leave it for 30 minutes to infuse off the heat and bingo, you’re ready to go. There’s plenty of variations around but use what you have to hand, and a slight fortifier of brandy will pair up perfectly.