You cut that into short lengths, and slide it over the metal pushrod. It then acts like a bushing to allow smoother sliding of the pushrod through the outter tube. If you use these (and you should), make sure you locate them where the one closest to the servo, and the one closest to the control surface will not travel out of the outter tube. If they do, they will bind the servo when they try to re-enter the tube. Just put them at least a 1/2 inch from the tube openings on both ends. Hope that made sense.
Then, spread them out evenly--one about every 3 or 4 inches or so. Once in place, hit them with a little thing CA, and they won't go anywhere. Just remember to cut them short enough that you'll have enough.

If I need to explain further, then please ask....
Good luck, and you made a good decision going solid metal!

Have you ever soldered metal pushrods?