Cat lovers frequently add new cats to their households. We get calls asking why it sometimes does not work. We wish the calls could come first, before the new cat arrives.
The following can both correct the problem after a failed attempt and smooth the way for an introduction.
Whenever possible, the new cat should be neutered and given its inoculations before being introduced. That way, you avoid an infection being passed on to your whole cat family.
Prepare a room for the new cat with a cat bed, litter box, water bowl, food dish, toys and scratching post. Make sure the windows are screened and the door can be closed. If you are moving into a new place with a cat you already have the room you will sleep in is the ideal "safe space."
Take the cat in its carrier into the room it will occupy. Close the door, sit down and talk calmly to the cat for five minutes. Open the carrier door. Continue talking and keep the cat company while s/he explores the room.
Have food and water there, but don't be surprised if they are ignored. When leaving the room, close the door. It may be a good idea to put a note on it saying, "Do not let the cat out."
Next day, take a small towel in with you. Visit with kitty, and gently wipe him/her down with it. Do this several times. Leave the towel there.
After several rubdowns, when you leave the room, drop the towel on the floor, outside the door. The resident cats will be very interested.
Next day, rub the resident cat(s) down with the same towel. Leave it on the floor by the door. Continue the rubdowns, and do not wash the towel.
After a week or two of rubdowns, the new cat and the resident cat(s) will be used to each other's scent. They will smell each other under the door, and may even talk. Two weeks is usually enough time to see acceptance begin. Scent is everything to cats.
When scent is accepted as normal, cats will coexist peacefully. At this point, open the door and watch what they do.
If peace and curiosity prevail, resident cats will watch / follow the new cat on its explorations of the house. When s/he returns to his/her room, close the door.
Next day, open the door again, and they will repeat the exploration. Close the new cat in when s/he returns to the safe space.
If at any point there is spitting, growling or outright attack, return the new cat to the safe room and wait a day or so before trying again. Continue towel rubdowns.
Total acceptance will occur when the alpha cat has established his/her dominance over the others. It will likely be your resident cat who dominated the others before. Rarely, it is the new arrival.
Once the key factors of scent acceptance and who is alpha are accepted peace will prevail. You may then leave the door of the "safe space" open.