I only have experience sewing down a double folded collar that was knitted via picking up stitches at the neckline and then binding off. In that case, you have a “braid” of bind off stitches and a braid of picked up stitches and you sew through those two braids. Example:
But I have just knit the Arctic Light Sweater from Kutovakika, where the original cast on row is at the top of the collar, you knit several inches of collar then you work the raglan. So there isn’t a braid of stitches at the bottom of the neck to sew the top of collar cast-on stitches/braid to. Photo:
How do I sew the collar down in this instance?