Do both sleeves at the same time. I think it is easier that way.
On the wrong side of the sleeve, mark the position of the solid line for the placket slit.
On the wrong side of the placket piece mark the solid line for the placket slit.
Position the right side of the placket piece face down on the wrong side of the sleeve. Line up the solid lines. The short side of the placket goes closest to the sleeve seam.
Place the left and right paper pieces on top of the fabric pieces, lining up the slit line with the markings.
Stitch on the dashed line around the slit.
Crease the paper along the stitch lines and peel it off.
Cut along the guide lines up the centre of the stitching.
Peel off the last of the paper.
Fold each side over the seam allowance and press.
Fold over 1/4" on each long edge and press.
Turn the placket piece to the right side. Press only the short top end of the placket rectangle.
Starting with the narrow side of the placket piece fold it over so the stitching line is just covered. Stitch it in place. Press.
Fold over the end to the snip. Press.
Fold the long side over to just cover the stitching.
Depending on which sleeve you are doing you will stitch up the long side and make a square at the top or make the square first then stitch down the long side. You can put a cross in the square if you like. Make the square 1/2" down the placket. This catches the underneath bit so it looks neat. Press.
When you stitch down the long side you will not stitch the under lap because it is narrower than the overlap.
Mark the solid line for each pleat with a pin.
Fold right sides together. Stitch 1/4" from the fold and about 1/2" into the sleeve with a short stitch.
Keep the first pleat out from under the second pleat as you sew.
Press the folds towards the placket.
This is the right sleeve.