sweater question-first sweater ever LOST IN SPACE!

[stitchwitch]

Neck Shaping
With RS facing, k across 14 (16, 18) sts.
Join 2 more balls of yarns.
Cast off 8 sts for neck.
K 14 (16, 18)sts.
Working both sides at the same time, dec 1 st at each neck edge on the next 4 rows - 10 (12, 14) sts on each shoulder.
Continue to work in st st until piece measures same as back to cast off edge.

Ok, I get the first part about knitting across the rows but I don't understand the whole joining another ball of yarn thing that sweaters require me to do. This pattern calls for using two strands of yarn so I get the "join two balls" thing because I'm using two balls for it, but I can't conceive in my little pea brain the whole idea of where the heck I join this yarn?! If I'm knitting along there is no area to just join another ball, right? I'm knitting along and I'm assuming "cast off" means take the stitches off the needle onto another needle. How does that leave an open end for me to join this? Or am I just being a complete dope?

[Ingrid]

What you're doing here is separating the two shoulders. Just knit across the first stitches with the strands you're using. Then, using two more strands, bind off those center 8 stitches and knit across the remaining stitches.
You don't to do anything special to join them. Just drop the two strands you've been using and start binding off those center stitches with the new strands. The binding off of the stitches will secure it.

Now you will have separate strands for each shoulder to work them higher than the back of the neck. Working them both at the same time means to do one side with whatever decreases you have to do for a row, and then doing the other side; turn and work back on one, then the other, etc.

[stitchwitch]

See, now I can visualize that!!! Thanks!!! Without your help I would so lost!