The whole tunic doesn't have any visible seams, except for the small sleeve. That meant that I had to find a fabric that was wide enough to provide full length sleeves, even if I just folded the fabric and cut a hole in the middle. I used a wonderful linen/rayon blend, after turn in the washing machine it's wonderfully soft, with a texture of linen but not as many wrinkles as a pure linen.
Then I folded it in half again, but in the other direction, and cut out an angular neck opening. The neck opening is in a diamond shape so no rounded corners.
To get the visible seam on the sleeve I made a dart at the top of the sleeve and pressed it flat.
The neck opening was strengthened with a piece of cotton tape.
The big challenge was to get the hem straight. In order to look straight it needs to be shorter at the sides than in the middle though.