First, your sleeve. There'll be a mark on the pattern piece for where and how long to cut the opening where the placket will be inserted. Note the measurement of the opening, because you'll need it when cutting your placket pieces.
I don't have any set-in-stone measurements for the placket - depending on what you want and where you're going with the shirt, it could vary. It's generally about 6 1/2" long for a man's shirt.
The under-placket will be the length of the opening, plus 1/4". The width can also vary, but I like 1 3/4." Wide enough to make a statement, not so wide it's showy. On a woman's shirt, you could probably go narrower, though I haven't tried it yet.
The second piece, the overlap, is going to be about 2 1/2" wide. As far as height, it needs to be 1/4" higher than the top of the finished placket, which is to say the peak or square over top of the opening, where you stitch to hold the whole shebang together. Another 2 to 3" is safe to work with. If it looks like too much when you pin it, then just shift the excess down to the bottom. You can always cut it off later.
For the overlap, fold it to the shape you want - see the first photo for what I did, which was the point construction. Leave 1/2" of the overlap the same height as the underlap (1/4" will be for the seam and the other 1/4" will be for turning the fabric over the seam). Before pinning the pieces to your sleeve, iron 1/4" seam allowances on the outside of both pieces and the top of the underlap. You won't want to do it after you sew; it'll be a complete pain. And shaping the overlap then will be impossible.
Easy to remember - the narrow underlap is for the narrower part of the sleeve; the wider overlap is for the wider part of the sleeve. For the first step, you're going to pin the pieces to the opening, right sides of the pieces to the wrong side of the sleeve. Stitch 1/4" from the edge and press.
Starting with the underlap, fold it out through the opening and over so that it covers the raw seam you just sewed. Pin it and stitch. The second photo is the the wrong side of the sleeve, with the underlap pressed over the seam, but not yet folded through the opening.
The third photo is the right side of the sleeve with the underlap folded out, ready to be stitched.
Next, on to the overlap. Fold it out through the center (the fourth photo shows the overlap pulled through the center, but not yet folded back or pinned into place over the underlap) and get it settled into position on top of the underlap.
Once you have the overlap secured on top of the under, press it nice and flat, pin it with more pins than you think necessary, and sew the box (or triangle, or whatever) at the top, making sure that you catch the top of the underlap in your seam. The fifth photo is the finished placket from the right side. Go slowly around the top of the placket - it's a little hard to judge the point sometimes, especially with a totally inadequate sewing machine light.
At this point, if you're going to put a buttonhole in the placket, you should do it You can, of course, add the buttonhole after the sleeve is sewn up and the cuff is attached, but there are a few more contortions involved.
The sixth photo is the finished placket from the wrong side. See how neat a finish this gives? I admit, I don't always obsess about the inside of my garments, but since my shirts are usually worn by someone else, I'll go a little further because I expect him to show them off . Besides, he spends a lot of time with his sleeves rolled up and the underside of the placket shows.
Here's the final shirt sleeve, cuff attached, topstitching done, buttons and buttonholes complete. I copied the pleating detail from one of his Italian shirts, which has 2 pleats on the overlap side and one small pleat on the underlap side. I didn't make my sleeve any wider, just tweaked the size of the existing pleats to make it work.
At the end I've included photos of a shirt I made him several years ago. The outside looks pretty good (though look at that soft and squishy interfacing; that was obviously pre-Pro-Woven) but the inside really shows the messiness at the top of the placket where there's just no way to turn the (cut-on) underlap and keep it flat when you sew on the separate overlap.
I'll admit, this is a longer method than the one piece placket, but it's one of those times where the extra steps give you a better result and actually take less time in the long run because once you get comfortable with this, it goes pretty quickly and if you press and pin everything before stitching, there aren't many ways to really go wrong. The top of the overlap gives a professional, clean finish, and you can personalize it by making it square, diagonal or whatever.
I also like the two piece placket because, as here, I can use two different fabrics. If I were using a stripe, this is where I'd turn it horizontal, or cut it on the bias for some interest.
Let's face it, a man's shirt is a man's shirt is a man's shirt. All that saves it from complete ordinariness is fabric choice and your level of finishing. This is a step up from most pattern instructions, and I feel like it's definitely taken my shirtmaking to another level. I've read quite a few books and every one has their own take on how to do this. While I love David Page Coffin's shirt book and think that overall he walks on water, this is still my preferred way of doing a sleeve placket.
Here's hoping that although long-winded, this makes sense. If you try it, please let me know how it turned out.