Knit fabric sewing has its pros and cons. It doesn't fray and stretch makes fitting more forgiving. However, fabrics like a rib knit or a French terry can bulk up really quickly and that presents problems for domestic sewing machines and twin needles.
I learned long ago while sewing baby clothes, there are tricks to making knits easier. For example: steam, press, then put a ham on top to let it cool in place works wonders. And: woven fabrics sewing doesn't apply to knit sewing. You can sew a coverstitch seam without a coverstitch machine. Most of us make the mistake early on thinking they need to sew rib binding like bias binding. You don't even need that second fold-under on the inside. You can skip that entirely. I'll be showing this tutorial on woven fabric so you can clearly see the contrast in fabrics and the stitches, but this is how I sew all of my knit binding.
After you have prepared your binding by pressing in half lengthwise, then also pressed in both seam allowances along the length towards center, careful to not press out center press, pin right sides to right sides around neckline as shown. While you won't be using the inside pressed seam allowance, pressing all lengths is helpful for fold and stitch guides.
Sew on around neckline in your preferred stitch. A triple stitch or a narrow lightning stitch is helpful here to keep your turn-of-cloth seam clean and not too bulky. I generally don't use a triple stitch on knits as I'm rough on my clothes, but it is popular with some.
Looking at the neckline from the wrong side of the garment, or the inside, press the edges in, over your opening or placket, then press the binding over the raw edge with the seam allowance past your first seam line. This envelopes the edge and will keep the opening neat and tidy. If you're sewing a neckline without an opening, then you're just pressing the seam allowance down.
Pin in place, perpendicular to the binding. You're going to sew from the right side, so you need to be able to pull your pins as you go.
Using your stretch stitch of choice, top stitch around the neckline, catching the seam allowance on the backside as you go. Here, I used my favorite 3-step zig-zag. In the black Shona Romper
at the top of this article, I used a twin needle. Both work much better and create a flatter result with only 2 layers of fabric from the neckband and the garment.
From the Shona Romper instructions, here are some knit stitches that may look familiar. Do you have a favorite?