How to sew a neck and onto a baby jumper

I’m finishing a Phildar jumper pattern and you have to knit separate wrist and neck bands. Ok so far but then it does not cast off but tells you to knit 3 extra rows in spare yarn. When you sew on the neck band do you catch one stitch at a time and then undo the three extra rows? Can’t imagine this. Sounds easy to drop a stitch! It’s 3 ply wool. Help!

Your idea of grafting one stitch at a time seems reasonable. Are there further instructions on how to attach the collar and cuffs? And would it be more straightforward to pick up sts at neck and wrists for the ribbing?

What is the name or pattern number?

It’s an old Phildar pattern 8719. It’s very odd- you knit it all in one, arms and all and then embroider 3 balloons on the front( secretly I wish I hadn’t started it but daughter found it and liked it so much!!!)

It’s very cute. I’d tend to pick up the sts and make the neck and cuffs easier. less fussy and maybe more stable