The shoulder extensions, caplets, how ever you'd like to describe them, have five details. Two are on the edge of the piece. I sewed less than a 1/4 inch seam on the edge, then one 1/2 inch in and another about 1/8 of an inch from the second row. I used the inside edge of the presser foot as a gauge for the 1/8 inch . Depending on your pressure foot and the markings on the sole plate of your sewing machine you can determine what you'll use as a guide. I will baste the sleeve in before dong the top stitching on the armhole. My current idea is to get the seam allowance, place the metal studs and then sew in the seam with a zipper foot. The concern is whether the studs will be secure through all the layers. As soon as I do a trial run I'll let you know.
The top of the photo above shows the interfacing in the collar section. It's a mid-weight sew in interfacing. It's possible a light weight would have be sufficient since the doubled fabric is fairly stiff. Not show is the clipping of the outside curving edge, clip out little "v"s so when it's turned inside out there will be a smoother curve.