The cable stitch is created by alternating the stem stitch and the outline stitch.
This stitch is worked left to right and the first stitch you take the needle will be on top of the line. The next stitch the needle will be on the bottom of the line. Make your beginning stitch as shown below as 1 and 2. Come up at 3 and go down at 4 (on top) and then come up at 5 on the bottom (near where you went down at 2). Make a stitch from 5 to 6 and continue down the length of the stitch alternating the top and the bottom stitches.