Start by buying a couple of scratching posts – these are just as satisfying to scratch as furniture, once your kitty understands what they are for. Place these in several spots around the house. Then, take your cat to a scratching post – most cats will instinctively know what it is for, but if your kitty appears confused, pick up her/his paw, and move it gently up and down on the post to prompt her/him.
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Keep a close eye on your cat – every time she/he uses the post, praise her and give her plenty of encouragement and treats. This will serve as positive reinforcement, and encourage the kitten to continue using the scratching post. In order to further attract the kitty towards the scratching post, rub some catnip onto it – any doubts your cat might have had about the post will be dispelled entirely.
However, every time you catch your cat scratching the furniture, some punishment will be required, in order to condition the cat to refrain from the destructive habit. Keep a squirt bottle filled with room temperature water handy, and every time your kitty decides to use her claws on the furniture, squirt her/him with this bottle. Couple this with verbal admonishment – you can sharply yell “no!” as you squirt, to get the message across.
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Continue in this vein for as long as necessary, rewarding every time the kitty uses the post, and admonishing every time she/he doesn’t. It might take a week or even a month, but gradually, your cat will become trained to use the scratching post, and stop clawing up the furniture.