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Keep the new cat in a separate room so the other cats can smell her through the door. You can use your bathroom if you don't have a spare room. Give the established cat something with the new cat's scent on it to sleep on. The ideal object is a T shirt with your scent on it that the new cat has sat on. This uses your scent as a "intermediary" and gives the established cat a gentle introduction. It may take a few days for your cats to not freak out or avoid your new cat's bedding. But over time, they should start to get used to the new scent. As a general rule, it is usually best to introduce a new cat when your existing cat is young. This will allow the two cats to get to know each other over a long period of time and hopefully bond as they grow. If you are planning to get a second cat or to add another cat to your household, it's important that you ensure they get along by introducing them properly. This means letting the cats see each other before they touch each other or interact. Consider placing the new cat in a cat carrier and placing this on the floor for the existing cat to sniff around and investigate, without the threat of being chased by the new cat. Alternatively, use a baby gate that is at least 36 inches high. Place it in the doorway of the new cat's room so she stays in her room and does not interact with your existing cat(s). Allow the cats to get a look at each other. Then, praise and reward both of them with a treat if there is no attacking or signs of aggression like hissing. Make sure the cats look at each other five to ten times a row, two to three times a day. Before you allow the cats to physically interact with each other, you need to ensure they have grown used to each other by smell and by sight. They should seem relaxed and calm when they look at each other and be okay with being in each other's presence for an extended period of time. If either cat starts to hiss, growl, or seem uncomfortable, move them out of each other's sight. Keep the baby gate door closed to the cats cannot interact with each other. Always quit while you're ahead and don't force the cats to get along. Patience is key as it may take some time for your cats to accept each other. Once your cats seem comfortable looking and smelling each other, you can start to encourage them to interact. Use a fishing rod type toy to play with the cats at the same time. You should also give each cat her own toy to play with. This will help the cats to associate being around each other with play time. If one cat starts to act aggressive, use the fishing rod toy to distract and redirect the cat. However, if both cats display any aggression or tension, separate the cats and return them to their separate spaces. You should never leave the cats alone to play until they seem comfortable and accepting of each other. If the cats play well together, reward them both with treats and praise. It's important to always reward both cats so they understand they are equal and there is no preferential treatment.
Allow the cats to smell each other before meeting. Let the cats observe each other before they physically meet. Pay attention to the body language of both cats. Play with the cats together.