I am knitting a baby’s sweater, which is worked in one piece, from the bottom of the back, up over the shoulders, to the bottom of the front. The left sleeve is worked as I knit, but the stitches for the right sleeve are placed on a stitch holder to be worked after knitting to the bottom front. My question is this: how do I join the new yarn when I begin knitting the right shoulder? I’m used to overlapping the old yarn end with the new yarn end and knitting both yarns for three stitches (like the knitting video teaches). However, in this case, there is no “old yarn end.” Thanks for any help. Connie
Just start knitting with it and you can weave the ends in later. If it bothers you after one row or so you can stop for a minute and weave that end in so it’s all finished when you finish the sleeve! 
It won’t unravel?
You aren’t joining one ball of yarn to another. You’re going back to pick up stitches that are being held until you can get to them. It’s a completely different situation, but it’s very easy!
Move the held stitches back on to your needle(s) and just start knitting, leaving a tail several inches long to be woven in later. It’s that simple!
Give it a try and see how it goes! Good Luck!
As for unraveling, weaving the tail into your work will take care of that!
Ok, I’ll do it! Thanks.
No, it doesn’t unravel; you only have one loose end and it’s somewhat anchored by the stitches you’ll knit for the sleeve.