Method from old Burda book, with personal experience added. Staystitch the slit for the placket on the exact seam lines. Clip to the corners (without clipping the thread of course). My slit is 1/2 inch wide.
Cut the two plackets. Width under placket 2 *0.5 inch + 2 * 1/4" seam allowance= 1.5 inch Upper placket 2 * 1 inch + 2 * 1/4" seam allowance = 2.5 inch
With right side of placket stitch to wrong side of sleeve. 1/8 inch seam allowance. Stop at top of previous stitching. The upper placket is placed at the side which has the most distance from the seam of the sleeve. Picture shows underside, but I prefer working from the right side, then you see the staystitching and can sew exactly on top of that. (Machine tension not quite good here)
Press 1/4 inch seams in plackets (could have been done before, I forgot.
Turn to right side of sleeve. Press the under placket, turn the triangle of the sleeve up behind the under placket. Edgestitch.
Make sure the triangle at the top is turned inside, like above picture.
Turn the upper placket and edgestitch to the corner (I'm a little off, just a sample)
Fold the top in the shape you want and edgestitch
Finished result on the inside.
