Help With Neckline of Sweater

Hello, I am knitting a sweater with a cowl neck. The pattern states that I make the cowl separate and attach later. The problem I have is the neck opening. I have done the first row binding off the correct amount of stitches. The second row says “purl to neck opening, using a cable cast on, cast on 21 sts, purl to end” I would appreciate some help with this. When I did the cast on, the yarn was on the wrong side from where I need to purl to end. Not sure what I did wrong.

Thank you!

OK, purl to the neck opening and then switch the needles in your hands. The needle with the working yarn coming off the tip will be in your left hand. Knit cast on the necessary sts and switch the needles back and continue purling to the end.
This video shows the idea although it’s a knit row instead of a purl row. No matter.