I've dealt with this situation many times over the years. I have nine cats now and have tried everything from putting them in a room by themselves and letting them sniff at each other under the door, to just letting them wander and see what happens.
What seemed to work best for me is that I take the new kitten and set him/her in my lap in the middle of the living room. The other cats would come around and sniff, maybe hiss and then eventually go about there business. It allowed them to get to know each other, while I'm there to intervene if anything happens.
If the kitten was very small I would keep him/her in the room alone with me at night so I could be assured nothing would ensue. During the day I let him/her explore the house, find the litter box and food and just kept a close eye on the others. Some were more obstinate than others and hissed and growled for about two weeks, but they all eventually got use to it and they became one big happy family.
My only exception was with my first cat. I had her for five years before getting another cat. Up until the day she died she didn't ever really warm up to any of them. She would let them sleep by her or walk by without incident, but playing and touching her was out of the question. It might have been that five years was just too long to wait before incorporating another kitten - or that she just had an attitude.
