Horizontal seam?

I’m knitting the “W” tank from knitty:

For the finishing, it just says to seam - but I’ve never done seaming where I’ve needed to seam the cast on edge to the bound off edge… if these stitches were live, I guess I could do kitchener, but what the heck do I do with stitches that are not live (does that make them dead:))… Thanks for any thoughts:)

Do it from the right side the way you would do mattress stitch, but pick up a stitch on each side rather than the bar.

so when i do that, do i ignore the cast on and count one stitch in from that (so i’m taking the yarn between the first and second inside stitches) or does the chain of the cast on (or bind off) count as the stitch?

I’d go one in from the edge–the first place where you can pick up two loops from a stitch.