Pinging you two - cat advice needed
I've tried different methods of introductions over the years and have found it easiest to just put the newbie(s) into the house with the others and let them meet face-to-face. I stay right there with them to monitor everything. I usually have an interesting toy or the laser pointer handy for distraction and interaction and that's seemed to help a bit.
Another trick I've learned is to pet each one and pet another and another....intermingling all their "smells".
There's going to be some hissing and fussing, no matter what. Cats are territorial by nature and the newbie will be seen as an intruder. With time, things will settle down.....to what degree and how long it takes varies.
With 44 cats, I've been through this over and over again. LOL!! Some of our cats are best buddies, others ignore each other, a few still hiss but none actually fight.
Good luck with this. There's really no sure-fire easy way.....it's a play-it-by-ear thing with cats, I've found. Contrary lil critters but I can't imagine life without them. :-)