My ten year old needs help with a sweater

My ten year old son has started a sweater project, but we are having problems with understanding the pattern. We are using this pattern

We are stuck at the part where you have to add another yarn ball. Is this just like adding yarn to change colors? How do you work both yarn balls with one needle? “Next row (divide for neck opening), knit 39 sts (45,50), join new ball of yarn, bind off next 4 sts, work rem 39 (45, 50). Working both sides at once, each on a separate ball of yarn, keeping 5 sts at bottom edges in garter st, center sts in stockinette st (k 1 row, p 1 row), and neck edges in garter st, work for 4 3⁄4 (5 3⁄4, 6 3⁄4) inches MORE, ending on a wrong side row.”

Is there a video online that can show me how to do this? I think I really need to see it. Thanks so much.

You’re going to work on each set of sts separately with its own ball. So knit across the first set, drop that yarn and pick up the one to bind off with, leaving a tail to weave in later. With that same yarn knit the other set of sts to the end of the row. Turn and go back across that set, drop the yarn, and pick up the other yarn to work on the other set of sts.

So, how do you join the other ball in then?

Thanks again for your help.

Just start knitting with it leaving about a 6" tail. The first couple sts may be a little loose for a row or two, but you can just pull the tail to snug them up and it won’t come unraveled. When you’re finished, weave in the tail the same as when you add a new skein to a project.