Problem with binding off in middle of row for sweater

Am trying to shape armhole for a sweater. The instructions say to knit the first 15 stitches for right shoulder, now drop the yarn you have been working with and join a new skein; with new yarn BIND OFF as to knit the next 21 stitches for the neck; knit across the remaining stitches for left shoulder. How do I join thread to bind off in middle of row? Any help of comments would be appreciated. This is my first attempt at a sweater.

To join yarn you just start knitting with it. It leaves a tail and if there is any hole or gap you can fix it when you’re done when you weave in the ends.