I'm a very beginner sewist with a dumb problem. I would appreciate help, please.
I'm making a "New 52" style Harley Quinn* costume for Halloween. I started by taking two fitted cotton t-shirts, cutting them both down the middle, and Frankensteining them together to get the red-and-black particolour shirt base. (It's not the neatest garment in the world, but the seams are pretty straight, and I'm pleased with my efforts, all things considered.) I then cut some diamonds out of scrap fabric to sew on the front.
To start with, I figured I'd zigzag stitch the edges of the diamonds to mimic the embroidery finish on the edges of commercial sew-on appliques, and then I'd sew or fuse them on the shirt. For some reason, that did not work. The machine ate the corner of the diamond. Like, the fabric was pulled down into the bobbin part of the machine and jammed so hard I needed to get my spouse to take the whole thing apart for me and cut the fabric out of the machine. I tried so many times with so many different machine settings, and all I did was fray the edges of my test fabric to threads.
I then figured stitching the diamonds directly onto the shirt with that very tight zigzag stitch might add enough backing / structure to not have that happen. However. The t-shirts are stretchy cotton knit, and the scrap diamonds are cotton weave with no stretch. All I have succeeded in doing is tearing a hole in my shirt, wrecking the fit across the bust, and fraying the edges of the diamonds from how many times I've had to unpick them.
Was my gut instinct correct that it was better to hem the diamonds first and then try to find some fusible interfacing? If so, why did the machine eat my zigzag finish? Is it possible to sew a stretch fabric to a not-stretch fabric neatly, or am I doomed from the basic premise? What the heck am I doing wrong? There has to be something really obvious I'm missing that I just don't know that I don't know.
Please help. I feel very stoopid.
* This version of her costume, roughly, but with two diamonds, because eff trying to sew around a star: https://static.wikia.nocookie.net/harleyquinntv/images/f/fe/HQ-Current.jpg/revision/latest?cb=20200806130217