Cats will scratch anything they feel like, which is why -- unless you don't like your furniture very much -- it makes sense to invest in a cat scratching post. This clever IKEA hack lets you make your own custom scratching post of any size.
The parts list for this hack are pretty simple:
You could use any old carpet offcuts or remnants -- in my experience, cats won't care -- and while you could buy a cheaper ready-made scratching post, it is unlikely to have the replace and repair potential of this particular hack.
A Stolmen Snudda cat tree [IKEA Hackers]