Turn your chain over and in the back you'll see horizontal "bumps" that resemble purl stitches.
With your working yarn, pick up stitches in these bumps. This will be the foundation of your cast on edge. If you used a waste yarn in a contrasting color for the chain, it should be easy to distinguish between the bumps and your stitches.
Pick up and knit stitches just as you would when picking up along a neckline, until you have the required number of stitches.
I prefer to pick up the stitches onto a crochet hook and then slide them (purlwise) onto the needle. It's an extra step, but it's the method I use for any kind of picking up and I just find it easier than trying to use a knitting needle.Once the stitches are on the needle, just work them as you would the stitches from any regular cast on, and follow your pattern instructions as written.