I'm a newbie to this forum too, and have only been through the process of introducing a new cat to existing cats once, so here's my experience, for what it's worth.
Our existing cats were two neutered males, then aged 1.5 yrs, whom we'd adopted together when they were 7 weeks or so old. We were introducing to them the approx 3 month old stray male kitten we inherited when we moved house last year.
Initially we moved the new kitten into the spare room, as all the how-to's I'd read recommended; set him up in there with a litter box, food, water, bedding and toys, and kept the door closed - spending lots of human time in there bonding with this very friendly, and no doubt grateful for regular food kitten. To start with, the new and existing cats were only aware of each others presence by the smell of the other(s) on our hands, and any scents and sounds that crept under the door and through walls.
On the third day we began to bring the kitten out into the living area for periods of 5-15 mins at a time, several times a day, in arms mostly, being stroked and played with a little. If either of the other two walked through the room and/or approached us and noticed the new boy, all well and good. The big ol' boys were naturally tentative at first, and tended to just look briefly then go on about their business, though clearly all cats were getting used to the sight of each other.
Around the fifth day we allowed new kitten to explore beyond the living room, including where the other cats are fed and where their litterboxes are, always keeping an eye on things, so that nofeline startled any other unduly. It's just not nice to have a relative stranger bust in on you when you're eating or relieving yourself, now is it?
On the first day that the new kitten was reasonably free in the house, one of the existing cats accepted him immediately, no questions asked. I spied them eating next to each other and sitting near each other in different parts of the house with never a hiss or growl. Wee new kitten still went off to his own room at night for another 2 to 2 /1/2 weeks, until I noticed that the 2nd of the big boys had stopped hissing and growling every time he encountered the new kitten. There were no actual attacks, as one or the other would just walk away after a bit of vocalising, for which I was thankful.
Well, that's probably more reading material than you were after, but that's the process of introduction as it played out in our house. I think things worked out relatively quickly for us, (it can take longer than that), but all signs at each stage were good. If bringing new kitten out into the living area had freaked out either of the existings too much, we would have left it longer before allowing new cat to begin to explore other parts of the house, etc. They all get along really well. Eating, sleeping and playing together, though I do admit to feeling super relaxed when, about a month into our having become a three cat household, new kitty was away for one night when he was neutered - but that was just about me taking awhile to trust that all truly was well.
Hope that helps, and is not just a repetition of everything else you've heard so far. Good luck!
Edited to add: Have just noticed that the stickied thread at the top of this page called "Behavior Problem - Look here first", has articles about introductions!