This is the style of archery I practice, clearly there would be more than one medieval style of archery, this one I took from the book "Bowmen of Crecy" with missing details filled in with what I have found to work. I do not claim that this is the best style to use, but it halved again my power and improved my accuracy to 98%.
1. Stand side-on to your target, feet perpendicular to each other and shoulder width (maybe a bit more) apart. Have a straight posture and remember to always keep your torso side-on as well, twisting reduces power and accuracy.
2. Raise your bow arm straight out horizontally to your side, keeping your elbow straight and your bow vertical. Turn your head to face the target.
3. Keeping your wrist straight, take hold of the string using the first joint (the one closest to your finger tips) of the middle three fingers of your drawing arm. The top finger above the arrow and the other two below it. The arrow should be resting on the same side of the bow your back is on (i.e. on the left side for right-handed archers).
4. Draw the bow by pulling your elbow backwards until the string reaches your earhole. This is called drawing to your ear. The standard draw length for a bow is 28".
5. Bend over slightly at the hips towards your target, keeping your head and arms horizontal.
6. Aiming consists of sighting along the top of your knuckles instead of along the arrow. This is because you are drawing beside your head instead of in front of it. If you sighted along the arrow you would be firing crookedly.
7. Loose (fire) by straightening the fingers holding the string. Do not let your drawing hand creep forward before firing.
That's it. If there are any questions or if you didn't understand any of the directions feel free to post your question.
*Historical Note* Medieval Archers would draw using the muscles in their chest, arms, back, even their legs. How they did this is unknown, we've lost the technique. Medieval bows also had draw weights (the amount of force needed to pull the string back to 28") of 80 - 180lbs. This is another thing lost as we cannot make replica bows using the same materials with the same draw weights. Both the technique and the technology used by those medieval archers has been lost to time.