Instructions
Create a separate space that you can close off from the rest of the house. Bring your shelter cat into her new home slowly by first allowing her to acclimate only to this one area. Keep your other cat (or cats) out of this area. Allow your new cat a significant amount of time to adjust to this new area first; be sensitive to your new cat's needs. A few days might suffice for some cats while others may take up to a week to adjust.
Divide your attention equally between the cat or cats you already own and your new cat for the next few days. Make each cat feel secure and comfortable, as well as loved. Keep your new cat separate until he feels at ease in its new area--once this happens you may begin the slow process of introducing the cats to each other.
Use a cat's many different senses to slowly introduce the cats to each other. Rub a piece of cloth or a cat toy on the cat (or cats) you already have. Bring this item into the area in which your new cat lives. Invert this process, bringing an item from the new cat's room with its scent and introduce it to the cat (or cats) already living in your home. Spend at least 2 days on this but allow a week for cats that appear to be skittish or shy.
Bring your new cat into the larger part of the house in a carrier. Place the carrier in a room where your other cat is. Allow the cats to investigate each other, but keep your new cat safe in the carrier. Allow some hissing and aggressive behavior--the cats must get this out their system in order for you to prevent further down the road when there is no protective carrier separating them.
Repeat this process in the opposite way: put your existing cat in the carrier and let your new cat roam around the house. This will allow your new cat to become familiar with the surroundings of your house in safety. Spend a few minutes on the first encounter and gradually increase the amount of time the cats spend in the same room.
Remove the carrier from the equation. Introduce your cats to each other while you are in the room. Create a calm environment by speaking in soft tones and petting both cats. Make the experience as positive as possible with toys and treats for the cats--this will create a positive association in the mind of each cat with regard to the other. Allow this to take as long as it needs to take; at this point you will have enough knowledge of your cats and their interactions to know if the meetings should consist of very short encounters frequently, or more sustained explorations of each other and life together.
How to Teach a Shelter Cat to Stop Hissing at Other Cats
Bringing a new cat into the fold of your already cat-filled home, even if that cat-filled home only has one other cat, can be a challenging task. Because cats are not by nature social animals (they do not run in a pack in the wild), it takes some preparation and sensitivity to introduce two cats successfully. If you are adopting a cat from a shelter, this task can be even more difficult. Allowing the cats to ease into a relationship will help limit any aggressive behavior the shelter cat may exhibit, such as hissing. The adjustment period can vary widely, ranging from 6-12 weeks.