Between the sections on "Fancy Stitches" and on "Irish Crochet" in the booklet from the American Thread Company (which I inherited from my MIL), there is a section on trims. The first 2 trims it talks about are cords.
The twisted cord is pretty self-explanatory. It's the second cord in the picture. To make this, you get several strands of string or yarn and twist them. Then you bring the ends together, and let the strands twist around each other. Finally, you tie a knot where the ends meet to prevent the cord from untwisting.
The other cord (1. Crocheted Cord) described in the booklet involves crocheting. (Directions for both cords are at the right.) It reminds me of a knitted i-cord.
Start out as if making a spiral hotpad. Then continue as you did in making the hotpad -- except do not make any increases. In the variation here, the stitches are crocheted through the back loop.
In particular, as before, "ch 6, join" means to join the last ch to the first with a slip stitch. Then "Work 1 sc in each st" is exactly how we started the hotpad -- round 1.
"Without joining rows" means do not slip stitch the last stitch of a round to the first stitch of the round -- in other words, work in a spiral just as you did the hotpad.
Finally, "working in back loop of stitch only" means to crochet into the back loop -- and not under the entire loop (both front and back loops) as we did when making the hotpad. This, crocheting into the back loop, leaves a ridge which spirals around the cord.
The cord in the last picture is made in Peaches & Creme and a size 7 or H (4.50 mm) crochet hook. It's about an inch across.