I don't use the cable cast-on much any more. I find that it lacks stretch, and is slow to work compared to the long-tail cast-on, which is far more versatile. That said, it has its uses and one of the best of those is for casting on to the end of a row of work-in-progress. If only a few stitches are needed, I'll simply cast them on by the backward loop method, but if more than a few are needed, as for a front or back neck in a top-down design, then the cable cast-on is the way to go.
Here's how. Start by inserting the RH needle between the last two stitches on the LH needle, like this.
Pull a stitch through the gap, working this first stitch rather close to the points of the needles to close up any possible gap in the space you're working into.
Pull the loop through and place it onto the LH needle from the bottom up, as if to knit, like this,
NOT from the top down, as if to purl, like this.
Now, place the needle between the stitch just made and the one before and continue as above, with the caveat that you don't need (or want) to work as snugly as you did for the first stitch.
Here's the wrinkle. After you pull the loop through for your last stitch, but before you put it on the needle, bring the yarn to the front, then put the last stitch onto the LH needle.
See how doing this last manoeuvre keeps that last stitch properly oriented?
This is what the cast-on edge will look like,
and it's just perfect for picking up stitches along a neckline.
However, before you do that, have a close look at the "cable". Notice that it has an upper loop and a lower loop.
Make sure you do your picking up into the lower loop, like this,
otherwise you'll end up with a line of purl bumps below your newly picked up stitches, instead of this nice clean line.
P.S. I apologize for the low light in the last three photos; they were taken very early in the morning. I think, though, that you can still see what's going on.