"Join yarn at armhole edge" what???

Hi! I’m knitting my first project, a sweater for my boyfriend. I’ve finished the first shoulder, BO and everything and the next move says to “join yarn at armhole edge of left side (as worn) and shape neck and shoulders as for right side, reversing all shapings as follows:” and then it says what to do.

Thing is, i have no idea what this means?! I haven’t measured out where the armhole should go (it says to do that further down the pattern) and how do I even join the yarn? Should I join a new ball where I ended the knit on the left shoulder on the other side of the neckline, or what?

Please help, I’ll be forever grateful!
This is the pattern:

Welcome to the forum!
Thanks very much for the pattern link. Go back to the held sts for the left shoulder. Slip those sts off the holder so that you have the right side (RS) or public side of the knitting facing you. The center sts are the neck edge so the opposite end is the armhole edge, even if you haven’t marked it. This armhole edge should be at the needle tip.

Hold a new end of yarn with about a 6’" tail next to the left needle and begin knitting these armhole edge sts. Don’t worry if the first stitch or so is loose. You can snug it up later. This is Row 1 under the left shoulder instructions. You can cut the yarn strand attached to the right shoulder leaving a 6" tail to weave in later or you can use a new ball of yarn for this.

You can always measure the armhole length later when you get ready to seam the sleeves to the body of the sweater.