I’d like to get a second cat. I have a new, much bigger apartment and I think TheMerchanCat could use a friend.
TheMerchanCat is five years old and incredibly sweet tempered. She has never ever swatted or even growled at me in the two years I’ve owned her, so I’m not worried about aggression on her end. She’s quite the attention whore, so I think she would like someone to play with while I’m at work.
My question: How best to integrate a new cat into an already catted household?
I have a system I have used which has worked well for me. For the first few days, I keep the new cat in a room with a door between him/her and the established cat. Litter and food/water are of course separate. This allows them to kind of get the idea of each other, smell that there is another cat in the house, before the first face to face meeting. The timing of the first meeting depends on the actions of the cats - if they are playing under the door it’s a pretty good signal that things are going to be ok. If they are growling I usually wait a bit longer. Once the time has come to open the door I just open it and observe, ready to rescue anyone who needs it. Don’t leave them together unsupervised until you are sure they are getting along.
I’ve never had trouble introducing new cats into the house. I always mean to use the techniques SnakesCatLady mentioned, but I never do. When Pippin mayherestinpeace 
first met Coppelia (not pictured here) and her now-with-my-parents siblings, he was mellow as could be. He was 13 when we introduced Tribble and 14 when he met Maya and was completely fine with them (maytheybothrestinpeace* )
Having had numerous cats of varying ages (from 15 to newborn), I’ve never seen any trouble with any of them. Except for Coppelia, the odd one–she doesn’t like anything with more than two legs. She’s outside most of the time, so it doesn’t really matter.
It could just be luck, so ymmv.
It’s been a bad year for cats in our family. We still have four left, though
Luckily for me, Seamus is an incredibly friendly-with-other-cats kinda cat. He won’t normally hiss until there’s a sign of aggression from the other cat, usually after Seamus bounds into the room with a “Hey! A new pal!” look on his face, and without any regard for the other cat just runs up and starts sniffing him/her. Some cats, or actually most IME, do not like this. But, even after another cat hisses or growls at him, he keeps coming back for more. He’s what we like to call special.
When I introduced him to Bridget he was quite interested in her, but she was completely aloof and acted like he was not there. Now, of course, they’re the best of friends.
A lot, of course, depends on the personalities and circumstances of the cats involved. If you’re adopting an older cat who’s been an only cat his/her whole life, s/he might not adjust well to having another cat around. A kitten just recently removed from his/her litter mates will most likely welcome another kitty. But then again, a middle-aged cat might not appreciate the new fuzzball perfecting his/her pouncing technique on him. The sex of the cats involved can be an issue as well. From what I’ve seen, two males pose the most problems, whereas male-female or female-female can usually be less troublesome.
But the advice already given is good. Defintely give them enough time to get used to each other before leaving them alone together, unless you know that one cat can escape/fit where the other can’t. I had a previous cat who was not AT ALL interested in Seamus’ friendly sniffing and the like, but she was a kitten and he was larger, so she just slept in my dresser, somewhere he couldn’t wriggle into. They got along great after a week or two, but unfortunately I have no pictures of the little Kiara because she died in a freak accident when her collar got caught on the chair.
Some hissing/growling/exploratory swatting is to be expected, even in cats that end up getting along great. IME, it shouldn’t take longer than a week or two for everyone to get to know each other, and the pecking order (if any) to be established.
