Part of what is supposedly good about sewing your own clothes is that you can adjust things to suit your own individual body, because the kind of person who has the body that is an exact size 12 or whatever is incredibly rare — human bodies are extremely variable.
So that’s good, but it still requires some skill. I spent a while looking at my bust here (I tried to give you two photos so you can see what I’m talking about, but it’s hard to photograph yourself for this kind of thing), trying to decide if I cared enough about the fact that my left breast is two-thirds the size of the right (due to cancer & lumpectomy) to try to do something about it in the swimsuit. I mean, I could, in theory, build in a shelf bra with some sewn-in padding? Maybe? If I knew what I was doing?
But then I decided to heck with it, which is what I’ve decided every time I’ve put on a swimsuit for the past seven years post-cancer, I just don’t care that much. Maybe someday I’ll learn how to do that, but for now, people can deal with lopsided me.
Then I went on to the next step, cutting elastics (and no, I couldn’t find the swimwear elastic I’d bought for this, so I used regular elastic, oh well), notching at the quarters per instructions, pinning the quartered elastic, and sewing it in. This is the most confusing part, if you’ve never sewn-in elastic before (different from the process of making a casing and inserting elastic), so if this is new to you, just take it slow and follow the instructions.
I had like twenty jams of my machine on the first pass around the neckline, some of them entailing actually unscrewing the plate so I could get my fabric out, which I would’ve been swearing about if I were the kind of person who swears, but I think I mostly was doing it to myself by trying to go too close to the edges. I moved in a little bit, gave myself a little more leeway, and it went much smoother after that. Still with the wrong stitch, but shh.
I used white thread, btw, because I knew I was okay with it showing up, even if the stitching was wonky and uneven (which it is), but normally you would match the color thread to your fabric, so errors like mine would be less visible. 
I’m pretending the wonky seams add to the homemade charm.
