Your required pieces for the placket are the shirt front, an under placket, a top placket, and interfacing for both plackets.
The interfacing should be the same length of the placket pieces, but half the width.
The under placket is slightly narrower than the top placket and approximately 1/2 inch longer than the neckline opening.
Attach the interfacing to the wrong sides of the placket pieces
I found it helpful to press the placket pieces in half prior to getting to the actual attaching to the neckline
Also, snip the neckline opening at the bottom, at an angle at each side corner
Okay, with the garment facing down, lay the under placket (also facing down) so it’s right edge is flush with the left edge of the neckline opening. The top of the placket should meet up with the top of the opening.
Sew in along that matched edge, ending 1/2 inch from the bottom of the placket, which is the actual bottom of the opening
Fold over and press the left edge of the placket 1/4 inch
Flip the placket to the front of the garment and fold it in half. The unsewn, folded edge should be just over the stitchline
Turn the right side of the garment down to the back, along with the under placket. You’ll have the clipped bottom of the neckline opening and the little 1/2 inch tail of the under placket sticking up.
Stitch these two little friends together.
and flip things back up. The tail will be sticking out on the front of the garment, but no worries, that will be taken care of in a minute
Please note: I didn’t use a pattern, just eyeballed the basic shapes of a shirt front and plackets. So things get a little puckery from here on out. If I had actually measured, or used a pattern, we’d have much smoother results. Unless your pattern calls for puckering. Hey! Let’s pretend like mine did! Okay, moving on…
Flip the garment back over so the wrong side if facing up. Take that top placket and you’re going to do the same thing as you did for the under placket, just on this side.
Fold over in half
And sew down both sides up to, but not past, the bottom of the neckline opening
Now just secure that bottom tail of the upper placket, covering up and finishing the under placket as you go.