A cable yarn is made up of two or more smaller plied yarns twisted together. The easiest cable yarn is a 4-ply, and it's a nice way to use up a bunch of singles you've got lying around. It comes out looking braided, an interesting texture effect.
To make this quick and easy, you need a way to make center-pull balls. That limits what you can start with because some yarns snarl if placed anywhere near a ball winder. Don't try it with a really fine single unless you know you can manage it. (If you insist, skein it first and set the twist.)
This can of course be done with bobbins or (very carefully) with hand-wound balls, if you don't have a ball winder or you are doing a lot of it. (There's only so much you can ply from a center-pull ball.) But that means you will almost certainly have extra bits hanging around when you are done.
Grab a likely single and inspect it. Will it survive the ball winder? If it's seriously lumpy or very thick or low twist, it might not. Determine the direction it was spun. It doesn't matter which, but you need to know. Now wind it into a center-pull ball. Only make it as large as you can easily hold in your hand. You will be plying from this and you need it to be able to stick your thumb inside and still manage the outside end with the other fingers. Take the two ends of the ball and hold them together in one hand. Push out the ballwinder core with your thumb, and leave it there. Yes, you now have a ball of yarn stuck on your thumb. It is keeping the ball organized. Most likely you have the inside end coming off towards your palm, so stick your thumbs together and push the ball to the other hand. Now you can pull both strands off the end of your very large yarn-covered thumb. (If you are not using a center-pull ball, you must have some way to control the tension. A good lazy kate will work, or coffe cans with lids. Poke a hole in the top, wind two balls and use one can for each.)
Ply from both ends of the ball, twisting in the direction opposite how the single was spun. This is looking like normal plying so far, but here is where it gets different. You want to add lots of twist, with a spindle that will take some effort and a larger spindle. If it were a normal yarn, it would be horribly overtwisted. Relax the overtwisted yarn and watch it snarl. Those snarls are what your finished cable will look like! Ply the entire ball like that, and note what those snarls looked like. That's how you know how much to twist in the next step. (If you really want to be precise, pull off a length and let it twist back on itself as a sample. If you plan to stop for a while, make a sample so you know for later. The twist will probably start to set while you are away.)
Wind the overtwisted yarn into a center-pull ball. You have to keep it under tension, and don't pull it off the ball winder just yet. Just undo the plastic core from the base. If you remove the core now, it will collapse into a miserable mess. When you are ready to begin, stick it on your thumb like before.
Ply from this new ball in the opposite direction as the last one. Add enough twist to make it look like those snarls you saw earlier. Skein, wash or steam to set the twist, and admire your new yarn.
For your next one, you can use up two leftover singles by overplying them together instead and then start with the second center-pull ball.