How to connect flat bottom up sleeves to body of cardigan

How-to Questions
1

I’m at the final stage - I just need to make up the cardigan.

I’m pretty confused about the steps for knitting the sleeves. The sleeves were knitted flat - after I attach them to the cardigan body, I will join the side and sleeve seams.

Mark points along side seam edges 20 cm / 7.75 in, either side of shoulder seams, then sew cast-off edge of sleeves to back and fronts between these points

Two questions:

  1. What use does the 20 cm measurement have here? I am essentially just knitting the sleeve to the top edge of the sides of the cardigan body right? I’ve inserted a photo of what I think I’m doing

IMG_5197 (1)
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  1. What kind of stitch do I use to join a CO row to sides (so not another CO edge but also not live stitches)? Would love if you have a link to a video tutorial
2

I like your diagrams!
You’re to seam the sleeve to the sweater body from those two x marks you’ve made on either side of the shoulder. The marks are 7 3/4" or 20cm from the top of the shoulder.
You can use mattress stitch if you like.