In the medieval era, tunics were a part of almost every man's wardrobe. The upper class wore tunics made of fine silk, while the rest of medieval Europe wore tunics made of wool. You may choose fleece for your tunic if you are looking for an easier construction job. Fleece doesn't ravel and requires no seam finishing. No matter which fabric you choose for your medieval tunic, almost anyone with basic sewing skills can create a tunic quickly and easily.
Choose a dress shirt that fits you well, but isn't tight. Lay your fabric over the back of the shirt, and trace the pattern with a fabric chalk or fabric pen. Ignore the added collar as your tunic will not have a collar, and do not include the sleeves. Add 1/2 inch around all sides except for the bottom to allow for seams. Add 7 inches in length to the bottom, all the way across. Fold your fabric over, so that you will cut through two pieces of fabric. You should have two pieces, a front and back. Cut the neckline down in the front by two to three inches.
Lay a sleeve, unbuttoned, on top of the fabric and trace along the top and bottom edge. Add 1/2 inch at both ends and along the longer bottom of the sleeve to allow for seams. Fold your fabric and place the longer top of the sleeve on the fold. Cut along the lines you've traced, but don't cut the fold. When you open the fabric you should have one opened sleeve. Place this on top of another piece of fabric and cut all the way around to have two opened sleeves.
Fold one of your sleeves in half, along the long fold you didn't cut earlier. If your fabric has a good side and a bad side, like most fabric, you'll want to fold your good sides in before sewing. If you have chosen a fabric that will ravel if unfinished, fold 1/4 inch up at the cuff area, pin, and stitch. If your fabric will not ravel, you can leave your cuffs unfinished. Fold your sleeve along the long fold that you did not cut earlier, wrong sides out. Pin well. Stitch the bottom of the sleeve, from the part that will be the underarm to the wrist. Repeat with the other sleeve
Take the two body pieces of the tunic, fold the good sides together and pin. Sew along the shoulders and sides, leaving the bottom, sleeve holes, and neck unfinished. If you are using a fabric that will ravel, turn the bottom of the tunic up 1/4 inch, pin, and stitch.
Attach the sleeves to the sleeve holes with pins. The seams that will meet at your underarms should be pinned first, then pin the top of the sleeve and shoulder together. Flatten these seams open before you pin so that there will be less bunching when you stitch. After this, the other pins should be placed evenly all the way around. Sew slowly around the sleeves at the pins.
If your neck needs to be finished, fold under 1/4 inch all the way around the neck and pin. If you are having trouble folding the neck under and it's bunching, you can ease the seam by cutting very tiny darts, less than 1/8 inch in length, every inch along the neck before pinning. Stitch the neck all the way around, being careful to flatten the shoulder seams as you sew over them. Turn tunic right side out, and check for any openings amongst your seams. To embellish the sleeves, you can choose to add leather ties or wooden toggle closures. To liven up the neckline, you can cut a 3 inch long slit. Your tunic should be worn above linen slacks or tights to make the entire costume perfect.
If using cotton, turn your tunic inside out and iron your seams open for a more polished look. Hand stitching is possible too, but it will take longer to complete by hand and your stitches will be less even. Considering the period you are imitating though, hand stitching will produce a more accurate representation.
Do not iron fleece. Do not iron over pins or they will melt on your fabric.