There is a right way and a wrong way to introduce a new cat into your home.
Cats are emotional creatures with big concerns about territory, and introductions are stressful for them.
From the perspective of your current cat, he is encountering an intruder. From the newcomer's perspective, he has just been dropped on hostile turf.
Introductions aren't easy, but there are some ways to minimize the trauma.
The sanctuary room.
You may even want to leave the cat carrier in the room with the door open, so the cat can venture out whenever he or she feels comfortable. The sanctuary room serves two purposes; it gives the new cat time to de-stress, and it also lets the resident cat know that only a small part of his territory is being invaded.
Everybody likes food, and cats are no exception. Don't be above using food bribery to encourage the cats to interact, as they need to learn that good things happen when they are in the presence of each other.
Feed them by placing bowls on either side of the closed sanctuary door. Let the cats and their comfort levels tell you how close to place the bowls.
The next step is to let the cats really smell each other. There are two ways to do this. First, you may want to try it with an object such as a clean sock. Take the sock and rub it gently along the newcomer cat's face to collect some facial pheromones, the scent chemicals that are released from the cat's scent glands. These pheromones are "friendly" pheromones, and cats rub them on objects in locations where they feel comfortable.
Place the sock in the main house and let the resident cat investigate it. Then repeat the process with the resident cat and provide that sock to the newcomer cat. You can do this multiple times, or, if both cats seem comfortable, you may want to exchange locations.
Let them see each other.
After the cats have become comfortable with each other's scents, you'll want to let them see each other. If you have a screen door, this is a great time to use it. If not, just opening the sanctuary door a crack during feeding time should do the trick.
Do this until both cats seem comfortable eating in each other's presence.
The final step is to let the cats actually be together. Before you do this, make sure you have provided lots of vertical territory, such as cat trees and perches, for them to access. This will greatly increase the amount of space the cats feel they have, and make them feel more secure in each other's presence.
Toys are also important during this phase, as they give the cats more appropriate ways to spend their time and energy than by fighting with each other.
As cat lovers know, you can never have too many cats. With these simple steps, you can introduce them without the headache.