But, since we're dealing with cats, not people, it's just not that simple, so you'll need to have some realistic expectations.
You need to move slowly during the introduction process to increase your chances for success. You mustn’t throw your pets together in a sink-or-swim situation and hope they'll work it out
Cats also dislike change, and a new cat in the house is a huge change. These two character traits mean you could have a tough (but not impassable) road ahead.
All of this means that your current pet and your new cat need to be introduced very slowly so they can get used to each other before a face-to-face meeting. Slow introductions help prevent fearful or aggressive behavior from developing. Below are some guidelines to help make the introductions go smoothly.
Be aware that the introduction process can take anywhere from a few days to a few weeks, or even a few months in extreme cases. Be patient.
Feed your resident pets and the newcomer on each side of the door to this room, so that they associate something enjoyable (eating!) with each other's smells. Don't put the food so close to the door that the animals are too upset by each other's presence to eat.
Gradually move the dishes closer to the door until your pets can eat calmly while standing directly on either side of the door.
Try to get your pets to interact with a toy. Tie a toy to each end of a string, then place it so there's a toy on either side of the door. Hopefully, they’ll start batting the toys around and maybe even batting paws.
Be sure to spend
Swap the blankets or beds the cats use or gently rub a washcloth on one cat’s cheeks and put it underneath the food dish of another. If there are more than two animals in the house, do the same for each animal.
When the pets finally do meet, at least their scents will be familiar.
Once your new cat is using her litter box and eating regularly while confined, let her have free time in the house while confining your other pets to the new cat's room. It's best to introduce yur new cat to a room or two at a time and increase her access to other rooms over a few days. This switch provides another way for them to experience each other's scents without a face-to-face meeting. It also allows the newcomer to get familiar with her new surroundings without the other animals frightening her.
You can do this several times a day, but only when you're home to supervise. If you have to leave the house, put your new kitty back in her room.
Next, after you’ve returned the cats to their designated parts of the house, use two doorstops to prop open the dividing door just enough to allow the animals to see each other. Repeat the whole process over a period of days—supervised, of course.
If you're really lucky (and your cats are inclined), they may do some mutual sniffing and grooming, and you're on your way to success. They may sit and stare at each other. You can provide distraction by dangling toys in front of them at the same time. This may encourage them to play together.
They might sniff each other, hiss, and walk away. That's to be expected. This may go on for a few days or so, and then you'll probably find them both sleeping on your bed.
Note: Never try to break up a cat fight by picking one up; You're bound to get hurt.
Have your cats examined by your vet before introductions to make sure they're all healthy. Have one litter box per cat plus an extra one.
Try to keep your resident pets' routine as close to what it was before the newcomer's arrival.
Make sure all cats have a "safe" place to escape to.