Here are photos showing you a technique I learned a long time ago for making a single crochet cord. It's a flat cord, with one side that looks like a double chain (the front) and the back is more like the purl side of a knit St st fabric.
I like to use this cord because it's fast to make (no turning your work!), and you can just keep going until you have the length you want (vs. pre-planning the length with a long chain, then working a row of sc).
Here's how it's done.
Ch 2, work 1 sc in second chain from hook. DO NOT TURN.
NOW, notice that the "post" of the sc you just worked has one strand at the far left (in the front). Or you may consider this the left side of the finished sc. Insert your crochet hook into that (picking up one strand). See the pink arrow below.
After inserting your hook there, this is what it looks like:
NOW, work a sc by yo, pull loop through (then you will be at the photo above, with 2 loops on your hook), yo, pull yarn through both loops on hook.
You've now done a second sc. Let's go through one more sc just to reiterate.
* DO NOT TURN. Find the far left strand of the sc you just worked and insert your hook just under it (just like you did above). See the pink arrow below.