First, if I may, a bit of semantics. To redact something is to write it out, write it down, edit it for publication or revise text. Certainly sometimes recipes are redacted by researchers of historic cookery, but we are usually re-creating or interpreting a recipe instead, particularly when experimenting with measurements and weird or obscure ingredients.
Now, when tackling a recipe that lists a bunch of ingredients, implies that the reader knows what 'appropriate spices' and such are, and otherwise is intended as a guide for someone who has done this before in an instruction period with their master or mother, I rely on my 'master' and mother. My 'master' is the collection of cookery books (modern and period) which I have accumulated and used in my lifetime, and my 'mother' is usually grandmother because as soon as I could reliably stir my mother pretty much stopped cooking because she hated it. My grandmother, however, raised five kids during the Depression, and trust me, the cooking education I got from someone who was rationed four pounds of meat a week for six people (she lost her eldest at age 5 in 1931) was invaluable for this sort of cooking.
I've come to the point where I try to ignore the 'modern' version supplied in a great many collections of period recipes. The book writer took liberties with the original. How they can translate 'sugar a grete deal' to one teaspoon is beyond me, and they sometimes seem to think we're too stupid to find out what certain ingredients are and find them or acceptable substitutes. I work from the original words, perhaps brought into modern English, and using the occasional help in figuring out what certain terms are (the online Middle English Dictionary and Anglo-Norman Dictionary are very helpful) so I can start by being as accurate as possible. Remember that what fails can meet up with a can of cream of something soup and become casserole or a batch of vanilla pudding or ice cream and become dessert, so don't worry about a lack of success. You won't waste any food.
Read the recipe. Then go take a shower or something so you can think about it a bit. Relate it to what you know about cooking, how to handle the various ingredients, dishes that use similar ones. Go back to the recipe later and read it again, noting whatever you missed the first time. Reconsider what's modern and similar. You'll begin to formulate an idea or proportions and measures. If necessary, look up a few recipes in modern books to help. Put the modern books back down, they're only for ideas.
Read the recipe again and begin assembling ingredients. Get half again more than you think you'll need of pretty much everything (you can always make a half-batch later or put it into a frittata, but not having enough will stop you cold.). Start playing with your food. Handle the packaging, read the ingredients list if they have one, look at it. Feel it, learn its texture, mass, scent. Get out utensils and pretty much every measuring thing you have, and lots and lots and lots of bowls. You'll maybe use half of them but if you want to dump two tablespoons of rice flour without adding it to the mix a clean bowl nearby is very handy.
Read the recipe again. Measure. Ballpark it, don't worry about leveling and all the minutiae of distressingly accurate modern cooking, just dump what seems right into separate bowls. Use what you know of cooking, your experience, and that little voice in your ear that whines, "that's too much salt" or "oh, come on, you like dates, add more!" or your 'mother' saying, "Too much liquid, it will never set at this rate." Check a modern cookbook for a similar recipe to decide what heat settings to use. Put the cookbook away again.
Make something. Whole batch, half-batch, whatever you feel comfortable with. While you're adding ingredients, watch it. If it looks too scant, add more. If it seems to be right and you still have half a cup, consider it right. Play with your food. If it's safe, taste it while you're working. Add all flavor-adding ingredients to suit your own taste. Don't omit a listed ingredient, but feel free to make it a token amount. Cook it.
Taste the finished product. Have friends in, SCA or modern (but warn them that it's 'foreign' so their palates won't get stuck on 'ewww' and negate the help), for a critique. Have stuff on hand to fix anything that's awful. Dead cakes make good syllabub and you can do amazing things with cream soup and a casserole dish with meat that's just not right. Figure out what you need more of, less of, none of, might consider adding.
Now you're ready to do an educated re-creation of that period recipe. Get more ingredients, learn from your previous experience, and make it again with alterations. I'll bet you it comes out good. Then write it down (redact it!) to save the information you've discovered.
It can take hours, days, or a week, but when you're done you've just taught yourself to cook in a period manner. And have one dish that's your own. And, hopefully, gotten over the worry that you'll be unable to achieve culinary pleasure for yourself and others.
I scared many of my kitchen crew by not listing measurements on spices and other augmentations used in the recipes for the Chateau feast. And the one dish that came out badly? The one I cooked all by myself without help from a set recipe. I burned the blancmange. Everyone who had to 'season to taste' and 'add until it looks/smells right' instructions came out with wonderful dishes that were met with pleasure and delight from the feasters.
Don't be afraid of your food. It and you are partners in life, and there is quite literally no 'wrong' way to re-create a period dish using the original ingredients. (Like all period cooks went to the CIA? Suuure.) And you'll end up having fun, too.