CATalyst Council, a group of animal health and welfare organizations working on behalf of cats, has a great list of tips to make the introduction a little smoother for all concerned:
1. Before you bring a new cat home, prepare a room where the newcomer can be comfortable and separate from your other cats. Ensure the room has a litter box, water and food containers, a comfortable place to relax and toys. Keep the door to this room shut for a day or so to allow your new furry friend to get used to being in a new place. This will also allow your existing cats to smell around the door and get used to the idea of a new cat in the house.
2. Periodically switch out a blanket or pillow in the room where the new cat is being kept with one from another room of your home. This will allow all cats to smell each other and get used to the others’ scents.
3. Slowly start allowing the new cat and your other cat(s) to see each other using a baby gate or other similar device, which will keep them separate and allow you to supervise their interactions. Praise the cats when they are being curious about one another (try using treats, a favorite toy or petting) to help them realize that their new friend is a wonderful thing and not a threat.
4. When all cats seem comfortable with each other, try allowing them to meet without the baby gate, but only under your supervision. Again, use treats (praise, petting, toys or treats) to reinforce how great it is to have a new friend in the house.
5. When they all seem to be getting along well under your supervision, allow the new cat to come out and explore your whole house without supervision.