https://www.cutoutandkeep.net/projects/how-to-sew-a-sleeve-placket • Posted by Pauline A.
Here is what your placket pattern piece should look like more or less. It should come with your shirt pattern but if not, you can find printable templates quite easily on the internet.
Place your placket piece over the sleeve, matching the cutting line marking. Important: both pieces should be with the wrong side facing up toward you! Stitch the box around the cutting line (follow the red line), pivoting at the angles. It's better to set your machine on a small stitch length.
Cut the opening line and finish with a V shape very close to the stitching line.
Turn the sleeve over and with right side up, flip the placket through the opening. Press open.
Take the small side and fold under along the folding line. Press.
Stitch very close to the folding line.
On the other side, fold the end forming a point. Press.
Fold along the two folding lines and press.
Now we have folded and pressed the fisrt line, we are going to fold the other one and press it.
Place the pointed part over the small one. We are now going to stitch the red line, enclosing the raw edges under the placket.
Edgestitch the placket following the red line. Go approx. 2 cm down the cuff opening to enclose the raw edge.
The result: nice and tidy! Now you can attach the cuff.