There are a few ways of sewing a shirt sleeve placket, but I prefer to use this method. Working with two separate pieces, called the underlap and overlap, is much easier then working with a one-piece placket. It also gives you the option to cut the two pieces from different types of fabrics, for a completely different look.
Step 1. Mark a rectangle 2cm wide and 14cm long on the wrong side of the sleeve and on the back of both sleeves, to avoid problems.
Step 2. Reinforce the marked rectangle with machine stitches and slash the center, and into the marked corners.
Step 3. Cut the overlap 8cm wide x 18cm long and the underlap 4cm x 13cm long. The excess can be trimmed off before completion. The length of the placket is optional.
This is the pattern for the overlap and underlap.
Step 4. Pin the right side of the underlap to the wrong side of back placket edge.
Machine stitch and reinforce the top corner of the placket.
Press and trim the seam allowance.
Step 5. Press the seam allowance toward the underlap.
Fold the underlap to the right side over the stitching line and tack in place. Edgestitch the folded edge and stop at the corner. Pivot.
Step 6. To construct the overlap, first mark it as indicated below.
Then fold it right sides together, pin and stitch around the top and down the side, up to the marked point. Turn it inside out with a point turner and press it.
Step 7. Pin the right side of overlap edge, to the wrong side of front placket edge. Stitch up to the corner and reinforce the top.
Step 8. Press the seam flat and trim the seam allowance. Press the seam allowance toward the overlap. Fold the overlap outer seam allowance and press. Bring the folded edge of overlap to the stitching line and pin it in place.
Note the upper corner of the placket and underlap is on the right side.
Mark a line at the top of stitching line with a removable marker as indicated below. This marks the position of the top stitching line.
Step 9. Tack the overlap in place.
Step 10. Topstitch (through all thicknesses) across the overlap and around the top of the overlap, down to the sleeve edge.
Step 11. Remove the tacking and press. Pleat the sleeve and attach a cuff.