It is relatively straightforward to get your cat to use a scratching post.
Your Cat Is Scratching the Carpet
If your cat is scratching the carpet you should buy a horizontal scratching board. There are many examples of this sort of scratching ‘post’. The photograph on this page is one example. It is made of cardboard.
They are inexpensive and about $15 (USA) or £15 in the UK. Also buy some catnip aerosol spray. Hold the scratching board and spray it with catnip. Do not spray the catnip onto the scratching board while it is on the floor because some of it will go onto the carpet which may encourage your cat to keep on scratching the carpet.
You should find that your cat is quickly attracted to the catnip on the scratching board and he may roll on it, sit on it and have an overwhelming desire to scratch it which will be a delight to see. Once he has started to scratch the scratching board/post, he will come back to scratch again and again. He should stop scratching the carpet. You might top up the catnip on the scratcher from time to time, perhaps every month.
Your Cat Scratches a Chair, Sofa or Wallpaper
This is when your cat scratches something vertical by which I mean he scratches the vertical part of the armrest of an armchair. In this instance I would buy a very sturdy, large scratching post. It must be sturdy and solid so that it does not move when scratched. This replicates a tree or something solid outside. Then use the catnip to spray the scratching post making sure that the catnip does not go onto other objects in the room (spray it outside).
Then place the post in the vicinity of the chair or sofa that he was scratching. That should do the trick.
Of course, there is no absolute guarantee that your cat will be attracted or motivated by catnip. Some cats aren’t but most are. I have found that these methods work for me. The snapshot photograph on this page is of my cat Gabriel who, as you can see is sitting on a scratching board, placed on the carpet that he was at one time scratching. I had just sprayed some fresh catnip onto the board and immediately he scratched it and then sat on it contentedly.
If anybody else has first-hand experience of this sort on subject, please leave a comment.