Firstly cut out the pieces.

Only cut a slit for the neck in one of the body sections (the front). If you want a better fit try angling the shoulder seams as shown rather than cutting them straight.

1. Sew up the shoulder seams and the top seams along the arms.
2. Fit and sew in the sleeves. Make sure that you get the selves fitted in centrally so that there won't be a twist when the tunic is sewn up.
3. Sew up the tunic from wrist to knee on both sides. I actually find it better to leave about 6 inches at the bottom and hem the edges to leave side splits rather than sewing all the way down to the bottom as this eases movement.
4. Hem around the bottom edge, neckline and cuffs.