Getting a new cat can be a difficult and trying task, especially if you have existing pets already in the home. Cats are territorial and hate change, which makes adding another cat a tough road to wander down. Just like people, you can't force two pets to live together. He best you can do is follow these simple tips to increase your chances of success:
Slow introductions help to prevent fearful or aggressive behaviors from all of the pets involved. By not forcing the animals together, you can minimize anxiety and apprehension by allowing them to approach and interact on their own terms.
Keep the newcomer in a small room with their litter box, food, water, toys, bedding, and other necessities so that they are separated from the other pets. By keeping them separate for several days, and up to a week, both parties will be able to smell each other through the door. You can even try feeding them on opposite sides of the door, or sticking a two-ended toy under the door and get them to play together under the door. This process will take time, and remember to take it slow. And be sure to spend plenty of time with both your new and existing cats.
By exchanging bedding, you can help your pets all get used to each other's scents. You can also confine your existing pets into one room and allow the newcomer to explore the house a bit as well, but only do this when you are home to supervise. Once you feel like both sides are gaining more comfort, you can use a small doorstop to slightly prop open the door, just so the newcomer and existing pet can see each other. Do this over several days to maximize comfort.
Remember that slow and steady wins the race. You can expect some tension and hissing, but as long as they walk away from each other, then the process is moving forward. There will be kinks for them to work out together, once they are face to face, but again that is to be expected. They might even start to play or mutually groom if you're lucky.
However, be on guard when they are around each other, and be on the lookout for signs of a potential scuffles, like staring and ears pressed back on their heads. In these case, break up their interaction, but never do it by picking one cat up because you will be bound to get hurt. You can also reduce any extra tension by placing multiple litter boxes throughout the house and making sure each cat has a safe to escape to if they feel overly stressed.