|Not your usual shirt placket
First, cut a piece for the placket. This needs to be cut on grain, the width is 1' (approx. 2.5 cm), the length is twice the length of your placket slit plus 1' (approx 2.5 cm).
On the wrong side of the sleeve, draw around the slit a rectangle, adding 1/4' on each side of the slit and a supplementary 1/4" at the top, where the slit stops (see my picture here - http://bp3.blogger.com/_e2XwlkD5wb0/R-DrMO-tOwI/AAAAAAAABDM/YY8r5xaOmgI/s1600-h/DSCN3137.JPG; look at the indigo lines, drawn with my vanishing marker. The white chalk marks are the original pattern marks. And sorry for the wonky lines, they are actually straight, but I moved the silk by mistake before taking the picture).
Now cut your slit open with really sharp scissors. You cut a straight line which is the length of your original slit (without the additional 1/4" at the top) and then, at the top, you cut diagonally to the top of the rectangle, like you would for a welt pocket. You can see the cut here - http://bp0.blogger.com/_e2XwlkD5wb0/R-DrMe-tOxI/AAAAAAAABDU/LFtH0xF1W1A/s1600-h/DSCN3138.JPG. I spread the sleeve so you can see accurately how I cut.
Now, take your placket and fold 1/4" on one long edge. Press. See it here - http://bp1.blogger.com/_e2XwlkD5wb0/R-DrLu-tOvI/AAAAAAAABDE/fOugAXfrKG8/s1600-h/DSCN3136.JPG
Put the placket's right side (the unfolded edge aligned with the slit to the sleeve's wrong side , and sew continuously: one side of the slit, the cut triangle, the other side of the slit. See the result here (the circle points you to the sewn triangle) - http://bp1.blogger.com/_e2XwlkD5wb0/R-DrMu-tOyI/AAAAAAAABDc/R0-tO2WHoE4/s1600-h/DSCN3139.JPG
Press your seam allowances towards your placket. It should look like this: http://bp2.blogger.com/_e2XwlkD5wb0/R-DrM--tOzI/AAAAAAAABDk/6gaeLFJomT4/s1600-h/DSCN3140.JPG
Now fold the placket and wrap it over your seam allowances, bringing the pre-folded edge to the right side of the sleeve (make sure you cover the stitching) - see it here: http://bp1.blogger.com/_e2XwlkD5wb0/R-Drzu-tO0I/AAAAAAAABDs/QZtv64cT6e4/s1600-h/DSCN3141.JPG
Press. At the top of the rectangle, form a peak with your finger and press in place. It should look like this: http://bp2.blogger.com/_e2XwlkD5wb0/R-Drz--tO1I/AAAAAAAABD0/yctbGV-a6k0/s1600-h/DSCN3142.JPG
Now you're ready to sew. Edgestitch (and edgestitching foot helps) the placket and you're done.
Finished placket - http://bp3.blogger.com/_e2XwlkD5wb0/R-DsGO-tO5I/AAAAAAAABEU/mctc742C2fo/s1600-h/DSCN3149.JPG
Wrong side of the placket - http://bp3.blogger.com/_e2XwlkD5wb0/R-Dr0O-tO2I/AAAAAAAABD8/pQ7XDIqrEbU/s1600-h/DSCN3151.JPG