I didn't find any info online. I'd imagine it was made by grinding up the nut, then mixing it with water. But how would they strain it then? Cloth was all hand made back when almond milk was first made, so did they just drink it without straining it, or did they skim from the top?
Your imagination is right. A mortar for grinding and a cloth for straining were the most often used tools.
Le Menagier de Paris’s almond milk soup
This delicate soup is ideal for the infirmary, but can also be enjoyed by healthy monks.
Bring a pot of water containing two onions to the boil. Meanwhile blanch about 300 grams of almonds in boiling water to soften the skins; leave to cool and peel. After the water has boiled for about 20 minutes, take out the onions and set aside, retaining the water. Next, pound the almonds in a large mortar or food processor, adding most of the water used to cook the onions. Strain the liquid through a muslin cloth to produce a fine milky liquid. Next fry the onions gently in butter. Once the onions are golden brown, add them to the almond milk. Serve with croutons or toast.
–– Andrew Jotischky: "A Hermit’s Cookbook. Monks, Food and Fasting in the Middle Ages", Continuum: New York, London, 2011.
Textiles are the solution to this. Cloth – like cheese cloth – for this was available for a long time.
If original manuscripts detailing this kind of recipe are more your thing:
Sion/Sitten, Médiathèque Valais, S 108 Parchment · 1 f. · 194.5 x 13.3 cm · second half of the 13th century Viandier Go to Overview Page
Language: French Manuscript Summary: This scroll contains a collection of 133 culinary recipes that served as a source for the famous Viandier of Guillaume Tirel, or Taillevent. It was part of the library of Bishop Walter Supersaxo (ca. 1402-1482) and his son Georges (ca. 1450-1529). (tog) Standard description: Description de Nadia Togni pour e-codices, 2010. Show standard description
Online Since: 11/04/2010
This is illustrated in