If you want only two major seams you will need a fabric twice the length of your gown so that you can fold it in fourths and cut the side seams only. If you use two pieces of fabric, you will have to join them, not only at the side seams but across the tops of the arms as well. Fold the fabric in half, placing your pattern on the fold and leave room for seam allowances. Don't forget to cut the neckline at the top of the fold opposite the sleeves. Another place that explains in depth how to make this type of tunic is at Ashlynn's Grove .
Tunics have a lower neckline than gowns and no sleeves.
Cut the neck out of the folded corner. Measure across your chest and divide this figure in half adding an inch or so to account for seams and movement allowances. Cut this far from the center of the fabric. Sew the side seams. These were the richest and most important people of the Middle Ages.