The tunic worn by the ancient Romans was a simple garment that looked like a huge pillow case with holes for the head and the arms. The tunic was worn as an outer garment by women and workmen and under the toga's of important men. It was a baggy garment that was given shape by the use of belts and pins.
Buy suitable cloth for the tunic. A natural colour wool fabric would be a good choice as this would be close to the fabric that would actually have been used.
Measure the person that you are making the tunic for. If you are making it for a man, measure from bicep to bicep and then down from the neck to just below the knee. For a woman, measure from elbow to elbow and then from the neck to the ankle.
Cut two pieces of fabric this size and put one on top of the other.
Sew each side up leaving a gap for the armholes. Then sew from the shoulders in towards the neck leaving room for the head to go through. For the most authentic finish, sew the garment by hand.