The Book says, “The Continuous Sleeve Placket finishes the raw edges of the opening with a separate strip of fabric.” it also says it’s common in women’s clothing.
I just did a rough sleeve pattern for this lesson. But basically what you’ll need is the sleeve piece, with a triangular cut out where the placket opening will be, and a 1.5” strip two times the length of that opening.
*note that the strip has been creased/ironed down the length of the center. this makes things a bit easier a few steps down.*
Flip the piece to the wrong side, pull the strip out. Press 1/4 inch of the unsewn side of the strip to the wrong side
Using the crease made earlier, fold the strip over to the front of the sleeve piece
Now take the bound sleeve placket and fold it in half, right sides together
Sew from the folded corner’s inside edge at an angle to the other side