I have never had to "Pick up and knit", so now that I'm starting a project that requires me to do so, I need a bit of help. I understand the concept of picking up and knitting, but how do I rejoin the yarn to start doing so??

I'm also thinking about trying out the hedgehog pattern from fiber trends and noticed "rejoin yarn to remaining stitches" in the pattern. Can someone please tell me what that means and how to do it?? I'd really appreciate the help.