Cut notches in the convex side and slits in the concave sides of the sash's curves. Cut off the sharp corner point. All this will allow the seam allowance to accommodate for the shape it will take once the sash is turned right side out.

Turn the sash right side out by pulling the ends through the hole you left open in the shoulder seam.

Use a chopstick to give definition to the points at the ends of the sash and get ready to press it.

Press your sash with the iron! You can close up the hole in the shoulder seam by hand, or just leave it as-is on the inside of the sash.