I really wonder who makes these things up. Or if a garment existed before the instructions were written. I'm working with vintage Simplicity 7882, copyright 1977, in a size technically a hair too small but actually a hair too large. I am so glad I'm doing a test run of this thing, because I would have lost my mind at this point had I used irreplaceable fabric. The dress bodice and skirt are a delight, but the facings don't fit. And the topstitched facings are what elevate this dress from pretty good to special.
I can just SEE the wheels turning. "We want lines of topstitiching for the sleeve detail, so let's cut it on the straight grain so they can follow the thread." Unfortunately, the armhole is curved. When you cut the facing or detail on the straight grain as a straight bit of fabric, you cause problems for a person with a sewing machine. An experienced seamster would read the instructions, laugh, and do it her own way. A trusting beginner would read the name of the pattern company, Simplicity, and assume it was going to be simple. Ho ho ho. A lunatic such as I will attempt to follow the instructions regardless of their idiocy.
We are instructed to sew the straight piece of fabric into a ring and then attach it to the bodice already stitched. This removes ANY chance you will be able to do it because fabric cut on the straight grain does not stretch, whereas the curved armhole does. Incidentally, the sewing term for armhole is armscye, a word I used to see frequently but now don't see at all. The sane thing to do would be to redraft the armscye to be a straight edge. I wanted to see how or if these instructions would work, so I left it as written.
Okay, we've gotten the ring made and now must try to sew it to the bodice. It would be easier to sew the flat piece to a bodice before sewing the underarm seams. I can see the wheels turning again. "Gee, it's really hard to get those short seams to align, and we want them to align on a channel quilted facing." It's as if a person with technical knowledge of sewing but no practical experience wrote these instructions. Or someone with no technical knowledge of sewing looked at the drawing and gave us a standard curved armhole despite the straight look of the drawing.
I used to assume these people spoke to each other and that I was wrong, or stupid, or inexperienced. At this stage of the game I've been decoding pattern instructions for 50 years, and I have a pretty good idea of what is possible and what is simple. This topstitching detail only looks simple. It is simple when the facings fit. It is simple when the fabric is wool. It may even be simple on a lightweight knit. It was not simple for soft cotton, and will undoubtedly be much worse for soft linen.
I caused some of my problems by not deconstructing after the test fit. With the skirt attached to the bodice, there was a lot of extra fabric to wrestle. When the pattern is new to you, it is wise to test the fitting before adding embellishments. You don't want to end up with a beautifully embellished thing that no one can wear. Or at least, I don't.
On the entirely rebuttable presumption that the instruction writer had actually done what he/she/it has written instructions for, I followed them. Note the lines of topstitching at armhole and neck. There are four of them. When doing that much topstitching, you should use a new needle. I did. You should also use a full bobbin; I did not. I have doubled threads for starts in the middle of more than one line of stitching. This is not lovely but not a disaster.
The single most important thing for facings stitched on top of the garment is that the facings should fit. It never occurred to me to check the fit of the facings. I didn't change the size or shape of the neckline or armscye. I ended up hand basting the front curve and adding an inch at the center back ends. Because the arm facings are straight, I just recut them.
Several lines of topstitching add stiffness to the fabric. Because this is a test garment and because of the topstitching, I did not interface the facing. I DID sew around the outside of the facing and iron it under, as suggested. When I sewed it to the dress, something happened on the curves: extra fabric. Because there are multiple curves in a low neckline, accidental ruffling was a strong possibility, even though I started my topstitching close to the last line of stitching. There was extra fabric, not stitched down. Oh, the horror.
When in doubt, fake it. So I added lines of topstitching on the extra wide curves. It does not look precisely like the drawing, but I'm not 6 feet tall like the model in the illustration, either.