Here's the tips I found online that I used with my three (aged 15 years, 10 months, and 6 months now).
INTRODUCING YOUR NEW CAT TO YOUR PETS
Cats who live in the same house may never become the best of friends, but usually learn to at least tolerate each other with a minimum of conflict. You will need to take some time to introduce your cat to other family pets in order to prevent fearful and aggressive problems from developing.
Introducing A New Cat to Other Cats
1. Confine the new cat to one room with its litter box, food, water, and a bed (done that!). Feed the present cats and the newcomer near either side of the door to this room. Don't put the food so close to the door that the cats are too upset by each other to eat. This will help start things out right by associating something enjoyable (eating) with each other's presence. Gradually move the dishes closer to the door until the cats can eat calmly directly on either side. Next, use two door stops to prop open the door just enough to allow the cats to see each other, and repeat the whole process.
2. Switch sleeping blankets between the new cat and resident cats so they have a chance to become accustomed to each other's scent. Also put the scented blankets underneath the food dishes.
3. Once the new cat is using its box and eating regularly while confined, let it have free time in the house while confining the other cats. This switch provides another way for the cats to experience each other's scent without a face to face meeting. It also allows the newcomer to become familiar with its new surroundings without being frightened by other animals.
4. Avoid any interactions between the cats which result in either fearful or aggressive behavior. If these responses are allowed to become habit, they can be difficult to change. It's better to introduce the animals to each other so gradually that neither cat becomes afraid or aggressive. You can expect mild forms of these behaviors, but don't give them the opportunity to intensify. If either cat becomes fearful or aggressive, separate them, and continue the introduction process in a series of gradual steps, as outlined above.
You'll need to add another litter box and scoop and clean all the boxes more frequently. Make sure that none of the cats is being "ambushed" by another while trying to use the box.
Expect hissing, spitting and growling. If a fight breaks out, do not interfere directly. Instead throw a blanket over each cat, wrapping the blanket around the cat before picking him up. Separate the cats until they have calmed down. It may be best to leave the cats separated when you are not home until you are sure they are getting along well. Successful introductions require time and patience. Don't expect things to be perfect overnight!
My own note; try to make each encounter as "happy" as possible with lots of praise for each kitty (it helps to have a friend around to help with this part of the process). Do you have a door in your home with a good gap under it? My kittens gradually got over their fear of one another by playing together on opposite sides of French doors with a large gap at the bottom. Considering the size difference between the two, it was amazing how cowardly the larger Oberon was towards the tiny Puck!
A product called "Feliway" (can be found at Petsmart) can really help calm your kitties nerves and help move the process along a little quicker. It took one week for each new cat, but take a look at the results!
Good Luck! And congrats on your new edition!