My Canyon came with a couple of lengths of thin plastic tubing. It’s similar to the tubing you get with something like Jagwire fully sealed cables.
All you do is disconnect the cable from one end (usually the mech) and remove the outer and ferrule where it enters the frame. Then push the tubing over the cable itself and into the frame void as far as you can. Loosen the other end and ferrule, then continue to carefully push the tube / pull the cable until the tube comes out of the other end. Make sure that the tube is exposed both ends of the frame.
Then hold the tube still whilst pulling the cable out. To replace, put cable through shifter, then through tube through the frame. Once cable is out the other side, pull the tube out.
Easy, quick, no faffing