Join yarn after BO?

I think I posted this question somewhere else accidentally. I am making the booties found here

On the button tab portion of the booties, it says to bind off all the stitches on my needle,then put the stitches from my stitch holder back onto the needle, join yarn and K 3 rows. How would I join the yarn? I am assuming I would cut the yarn after the BO. What then?

Either cut your yarn, or take the other end of the skein. Then just start knitting with it, leaving a tail long enough to weave in later.