Kittens are easiest to teach.
You have to discover what texture most appeals to your cat. Wood, carpet, sisal, cardboard, or even a variety of them. Covering the illegal scratching place with double-sided tape or with adhesive shelf paper sticky side out will provide a deterrent when you're not home. At the same time you're teaching your cat where it's allowed to scratch, you have to get it used to you handling its paws, and used to seeing the nail clippers. Start off by holding one of the cat's front paws briefly during a petting session.