Real milk, butter, and cheese are mentioned in
Redwall, so I have no qualms in using them. However, in all of the later books, it's always called greensap milk. I don't know what that would be, exactly, I think it's just a term used to indicate that it's not mammal milk. So I've gone with something that makes sense to me and is historically justified, which is nut milk.
Almond milk seems to be the most common in medieval recipes, but I've heard of other types of nut milk as well. I've even heard of oat milk, which I will try to make sometime with this method, using whole oat groats.
The nuts you can use are almonds, cashews, hazelnuts, hickory nuts, walnuts, and pecans. The first three are the more popular choices because they're not as bitter.
For a much prettier and more detailed tutorial on how to make almond milk, go
here. The recipe there also tells you about using sweeteners and spices in the milk.
Now here's my more basic recipe. You can use any of the nuts above, as stated. For simplicity I'm just going to say almonds the whole time.
This makes about 6 cups.
1.5 cups raw almonds
water
Blanch the almonds if desired. The skins make it taste slightly bitter, but I don't usually blanch them because I'll be using the milk in recipes that use honey, so the bitterness doesn't come through.
To blanch them, boil about 2 cups of water. Pour in the almonds. Boil for about 3 minutes, then strain out the water and let the almonds cool. Rub off the skins by hand.
Cover almonds in water and let soak for at least 4 hours. When they are soft, strain off the water and place almonds in a blender. Add 1 1/2 cups of fresh water. Blend until they have formed a smooth-looking puree. Add 2 1/2 more cups of water and continue blending until it is smooth and frothy, 2-3 minutes.
At this point you can strain the milk. If you are planning on drinking it straight, it's more palatable to be strained. For use in recipes, it doesn't have to be strained.
Place 3-4 layers of cheesecloth in a fine mesh strainer, over a bowl. Pour the almond milk through. When most of the liquid has gone through, bundle the cheesecloth and squeeze out the last bit of liquid. You can dry the almond meal that remains, which can be used as almond flour.