It can be challenging to add a new cat or kitten to a household that already includes one or more cats. Patience is key to the transition as it may take several weeks for cats to accept one another. The first step is to isolate your new cat or kitten in a separate room. He should have his own food, water, litter box, toys and bedding. When all of the cats in the home seem relaxed, over a period of days start to slowly move the food dishes for all cats closer to the door that separates them. If there is any evidence of stress, go back to the step where they were comfortable and work up again more slowly. Offering toys that they can play with under the door, using Feliway diffusers and collars and rewarding calm behavior with treats can help everyone become more comfortable.
Once the cats are relaxed with this arrangement, gradually start allowing them to see each other through a cracked door, screen door, or baby gate. Work up to letting the cats trade places for short periods (i.e. putting the original cat(s) in the isolation room and allowing the newcomer to start exploring the house). If the cats are acclimated to wearing a harness they can be used during initial face to face meetings so the cats can be safely separated if needed. Once the cats are comfortable with each other maintain multiple food, water, and litterboxes throughout the house to ensure everyone has free access to these resources.