The SCA slang for this process is "redacting" recipes. There are a couple of things you can do to prepare. The first is to cook medieval food from recipes other people have redacted. I like the recipes page at Medieval Cookery, which is consistent about including the medieval recipe along with the redaction. I've also cooked a lot from The Madrone Culinary Guild's pamphlets and The Medieval Kitchen: Recipes from France and Italy. (A friend of mine actually learned to cook from this book, because it was the only cookbook she owned as an undergrad.)
( More advice, and a walkthrough redaction! )