Source: Le Menagier de Paris, but see Note 1 below
For 500 ml. of wine: * 500 ml of red wine * 1 tsp. powder douce * 1-3 tbsp. sugar
For the powder douce (see Note 2 below): * 3 tbsp. ginger * 2 tbsp. sugar * 1-2 tbsp. cinnamon * 1 tsp. cloves * 1 tsp. nutmeg
Mix the powder douce ingredients. Take 1 tsp. of it and mix it with the wine, along with an extra 1-3 tbsp. sugar and stir until the sugar dissolves. Serve cold or hot, like mulled wine.
Note 1: This recipe appears in many cookbooks, in all sorts of ways. Le Menagier gives the most accessible version, but it's clear that there was a great deal of regional variation with this recipe. In fact, Le Menagier gives an additional recipe ("by the measure used in Besiers, Carcassonne, or Montpelier") which is only slightly more ellaborate.
Note 2 Powder douce is used in a lot of other recipes. It's almost a generic term and it was, undoubtedly, adjusted by every cook, according to regional and budgetary constraints.
Note 3 The recipe above is the "accurate" one, as in, it's the one that Le Menagier gives. Having heeded Note 2 above, I use a slightly different version: I use anise and some extra cloves for the powder douce, and substitute sugar with honey, which I mix in along with the spiced base when I need it.
From this translation:
HIPPOCRAS. To make powdered hippocras, take a quarter-ounce of very fine cinnamon, hand-picked by tasting it, an ounce of very fine meche ginger and an ounce of grains of paradise, a sixth of an ounce of nutmeg and galingale together, and pound it all together. And when you want to make hippocras, take a good half-ounce or more of this powder and two quarter-ounces of sugar, and mix them together, and a quart of wine as measured in Paris.
And note that the powder and the sugar mixed together make "duke's powder".