Cable cast on

From BluWiki

You start this the same as the Knit on cast on. Tie a slip knot onto your needle. Insert the left hand needle into the loop in the same way the right hand needle is addressing it.

Wrap the yarn around the needle as if to knit.

Pull the yarn through as if to knit.

Place the new loop on the right hand needle. That gets the first two loops onto the needle. Things get a little different from here. Put the needle between the two stitches.

Wrap the yarn as if to knit.

Pull it through.

And put the new loop on the right hand needle.

The video demonstrating this is here.