She said she casted on 100 stitches for the neck opening. I was thinking maybe to swatch with the needles of my choice and then measure around my neck to find out how big the opening should be, and then CO enough stitches to equal that many inches?
Hell, I have no idea what she means here.
Yeah, you have the right idea. That's exactly what you should do. Just make sure, when you rib, to use needles that are at least a size smaller for the ribbing. Her needle list, I think, was just to show you what she used. So if you decide to knit them on size 13 needles and a bulky yarn, go down to size 11 needles to do the initial ribbing.