So you've been considering bringing another cat into the family. The difficulty of this action will depend entirely on the cats involved. Some cats will need to be acclimated more slowly than others and there are cats who will wish to remain solitary no matter how carefully you go about introductions. But there are some things you can try to mitigate any potential problems. Keep in mind that cats are highly territorial so have patience.
Ready a room for the new cat prior to bringing him home. Select a guest or spare room where the new cat can be confined and kept away from resident cats. Set up a litter box, clean litter, fresh water and food. Put some toys and catnip in the room as well. This should be a room with a door that closes to ensure no other animals can enter.
Before you bring the new cat into the house make sure that all resident animals are current with their vaccinations. Also have your vet examine the new cat to make sure he is in good health and vaccines up to date.
When you arrive home with the new cat, take him straight into his new room and shut the door. Open the carrier door and let him come out as quickly or as slowly as he wants. Don't try to coax him out or tip the carrier to force him out. You need to let him build trust in you and the environment on his own terms. Allow the cat to get completely comfortable in this room first before expanding his range.
Encourage the cats to play with one another under the door. Most doors have an inch or so gap underneath them. Using a feather or cat toy, entice the cat on the other side of the door to play. Allow them to use their paws to play with each other under the door.
Place a towel, cloth or blanket in the bedding area of the new cat and vice versa with your cat already in residence. The purpose of this is to get the scent of each cat onto the item. Around the third day switch the cloths. Place the resident cat's cloth inside the room with the new cat and vice versa. This will allow each cat to start getting used to the scent of the other cat prior to their meeting.
On the third day, shut your resident cat (and any other animals) into a room and open the door to the new cat's room. Just leave the door open for an hour or two and let him emerge on his own time. Do this every day gradually increasing his time out. When you believe the cat has adjusted to the primary areas of the house, thus gaining a sense of security, it is time for introductions to begin.
Introduce the cats to on another. The introduction should take place in a room that is comfortable and familiar to both cats. If you need to introduce the new cat to more than one cat, do so one at a time. Provide the new cat with an escape route or hiding place, such as an upended cardboard box with a hole cut in it.
Bring both cats into the room and set them down in sight of each other. Do not intervene unless they start hissing and growling at each other. In that case, remove the new cat to his room and try again the next day. If the hissing persists after a couple of attempts you may have to place the new cat in a crate or vertical cat condo.
If using a cat crate or vertical cat condo, place it in a high traffic area where the new cat will be safe inside but the other cats cannot get at him. He will feel safe and be able to watch the daily comings and goings of the household to become accustomed to the daily routine and the resident animals. Over time, you can let the new cat out and only use the cat condo in the event of an attempted cat fight or when you are not home to supervise.