Stolen from a staffi site:-
The most important thing for owners to know is that this adjustment is a process, not a one-time introduction. Care must be taken to introduce dogs and cats slowly, making the process as stress-free and pleasant as possible. Owners must then be prepared to manage their pets’ interactions for the next several weeks, if not longer.
While careful introductions are the best way to set everyone up for success, there is no guarantee that your cat and dog will become buddies. Some pets learn to tolerate each other, while others might attempt to cause each other harm. The outcome will depend both on the manner of the pets’ introduction and their individual personalities. The following steps can help to maximize your chance of long-term success.
Keep the pets separate for at least the first 3-4 days. Prevent any contact until your new pet has had his vet checkup and been cleared of illness. Some families confine their cat in a sanctuary room with the door closed, others confine each pet to a separate floor of their house, still others confine their dog in a finished basement. The goal is to allow the pets to get used to each other’s presence without face-to-face contact. Even if they can’t see each other, they can hear and smell each other.
While the pets are still separate, begin to feed them on opposite sides of a closed door. The idea is to teach them to associate the presence of the other pet with pleasant things, such as food. With each feeding, move their food bowls a little closer to the closed door; continue the process until each pet can eat calmly right next to the door.
Begin teaching your dog basic obedience cues, such as “sit” and “down”. Keep training sessions short, pleasant and rewarding for the dog.
When the pets can eat their food calmly right next to the door, begin face-to-face meetings. Keep the first few sessions short and calm. Keep the dog on a leash and let the cat come and go as she wishes. Do not restrain either pet in your arms, as injury could result (if either pet behaves aggressively). Ask the dog to sit and reward him with small, tasty treats for calm behavior; toss treats to the cat as well. If either pet demonstrates aggression, calmly distract and redirect them: toss a toy for the cat to lure her from the room, call the dog’s name and reward his attention. Return the pets to their confinement areas.
Repeat these face-to-face sessions daily, saving the pets’ favorite treats for when they are together. If the cat attempts to leave the room, allow her to do so, and do not let the dog chase her. Try to end each session before either pet shows stress or aggression. (Conduct these “meet and greets” in common areas of the house; do not use either pet’s sanctuary room.)
When both animals appear to be getting along well, allow them loose in the room together, keeping the dog’s leash attached and dragging on the floor. (This will allow you to step on it and prevent the dog from chasing the cat if he gets excited.) If tension erupts between them, go back to the earlier introduction steps and repeat the process. Make sure the cat has access to a dog-proofed sanctuary at all times, complete with a litter box. Continue to separate the pets when you are not there to supervise.
If one aspect of this process bears repeating, it’s that first impressions matter: introduce your pets slowly and carefully to maximize the chance of long-term success. Good luck!