I currently have a 2 year old female cat. I have got a new 4 month old female cat today. How do I introduce them best?

My current Cat does not go out side. She is an indoor cat. She has come into contact with other cats before (at old home she went outside) She just hissed at them and the other cat would tend to leave her alone.

So what is the best way to introduce the new cat to Fluff?

I have 4 cats and tonight I’m going to get another cat (not kitten) from a friend who can’t keep the cat. Does anyone have good suggestions for getting the cats used to each other quickly? How can I convince my existing cats to get along with the new cat?

My family adopted a cat about two years ago. She has a chronic upper respitory infection. We are considering getting a second cat, but we are worried that the new cat will catch our old cat’s illness?