I let them get used to each others scent, they played with their paws from under the safe room door for five days. Then I put the new kitty in a cat cage and allowed my cat Frankie to get to know her. Followed by the introduction with supervision. The face to face introduction went better than I had anticipated.
The new little kitty was territorial and I intervened, by grabbing both cats by their scruff, the way a mother cat would do. Then I petted both cats while speaking to them softly, followed by click from my clicker and a Temptations cat treat.
Cat treats help because both cats were more interested in how to get another treat from me and did not think about getting to know each other. Food is your friend when introducing new cat to existing cats.
The first afternoon for introductions was uneventful, the new female followed my male cat and my male cat followed the new female. Now things did get a little testy when the both tried to get on the cat tree at the same time. It was a territorial thing and I modified that by spraying the entire tree with catnip.
We broke it up, and thought maybe our introduction was not slow enough so new female cat went back into the cat cage. That is when my cat Frankie climbed up on the side to give her a kiss. So we let her out and it has been love ever since.
One cat roll and tumble and they were done. Now both cats seem to be getting along.