I learned this from Connie Amaden-Crawford, and it's very nicely explained and illustrated in her sewing book (Guide to Fashion Sewing).
The instructions (right and left) are for a woman's shirt... for a man's shirt, just swap right and left sides.
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This is easier to understand if you've got a commercially sewn shirt in front of you:
**The standard 1.5" shirt placket, and how to draw it.**Here's the right side placket, and how to construct it:
-Make two copies of the front pattern piece, adding a 2.75" extension to the pattern piece from the center front. Mark one of them "Right side".
-Start with the CF line of the shirt. Notch it top and bottom.
-Draw two lines 3/4" on each side of the CF line. That's going to be the actual shirt placket.
-Measure over 1/2" from the line that you drew that's farthest from the shoulder. Notch it at top and bottom, too.
-Measure over 1.5" from that line you've just drawn (away from the shoulder), and draw one more line.
That's your right front pattern piece. Now to sew it up:
-Cut a 1.5" wide strip of fusible interfacing, and fuse it to the wrong side of the fabric, right at the edge.
-Fold the fabric to enclose the interfacing (you'll be folding on the notches farthest from the shoulder). Press.
-Fold once more, same direction, at the second set of notches (CF line). press.
-Sew 1/4" in from that last pressing, catching all layers.
-Open the placket out flat and press -- this gives you a 1/4" flapdoodle on the outside of the shirt, toward the shoulder.
------- Now for the left front placket -- if you look at the second copy of the pattern piece you've made, it's exactly the same as the right side (so you get to cut the front of the shirt double! Only thing that changes is the position of the notches for the left side placket. But just for completeness, let's draw a separate left side.
--Grab that other pattern piece, and label it as the left. --Measure 1/2" in from the cut edge, and notch it top and bottom. --Measure over 1.5" and notch it top and bottom. The CF line is still 3/4" farther on from the last line you've drawn here.
Sewing the left placket (underlap). -- Cut another 1.5" wide strip of interfacing and fuse it to the wrong side of the left shirt front, 1/2" over from that cut edge. (Yes, there really is 1/2" of un-interfaced material on that edge) -- Press the cut edge over the interfacing... so you're turning that 1/2" of material over the interfacing. -- Roll the interfacing over, so you're pressing at the second set of notches. -- Topstitch at 1/4" in from both edges of the placket. Done.
Once you figure out the roll under and stitch bits, it's an incredibly fast way to make a shirt placket. Turn, press, turn, press, sew, press and you're done! It's really amazing! And if you're working with something that has a horizontal pattern to it, like my beloved plaids, IT ALWAYS MATCHES!!!!! YAHOOOOOOOOO!