At uni on Friday there was some trouble with a placket. Someone was using a method I had never seen before and it seemed to make the job difficult and give a messy result. She was trying to insert the placket in a seam and it just didn't work out easily. At the time I couldn't remember the way I do it to show her, so I worked it out again last night. As with everything, it's the pattern that makes the difference, followed by the method.
Here is the pattern:
Perhaps you can see how the pattern was worked out. I don't like the wide plackets that are always in instructions; I want a nice, slim, feminine placket if I have them on my clothes. So I drafted one with a 1cm hole. That means that the seam allowances in the slit will be 0.5cm. The underlap will have to at least wrap around this seam allowance and turn under to be stitched down. So it will have to be at least twice the seam allowance on the slit (0.5cm x 2 = 1cm) plus 0.5cm "hem allowance", giving 1.5cm from the slit's stitching line.
The gauntlet (the bit you see with point on its top) will be the width of the slit plus 0.2cm overlap. It needs a cut-on facing (meaning it's doubled/mirrored) plus a seam allowance of 0.5cm. There is a 0.5cm seam allowance around the top of the placket too, and a 1cm seam allowance at the cuff edge.
The measurements of the placket can vary as you wish. I think the underlap could stand to be wider than I have drafted it to better accommodate the buttons. I wouldn't have the slit much narrower though; you need to have a the two seam allowances in it. The buttonholes should also be marked ahead of time according to the buttons you are going to use.
Here are the sewing instructions I sketched last night:
I was going to do a full tutorial, but then I found this one online which is so much better. The placket is different and I might do my own tutorial later, but I think I should do a neater sample if I do because mine is not very neat. It's simply not as good as the one on the Shirtmaker's Blog. If you want a one-piece placket, maybe you can make out what I've written and drawn on the above scanned page for now. : )