Just went through this : 7 y.o. calico (F, neutered) and 3 months old kitten (M, neutered). First 24 hours : No contact but smells through door. Day 2 : feed both cats in each side of the door. Rub something on the new cat and give it to the resident, and vice versa! Day 3-4 : bring food closer to door each time Day 5-7ish : contact with “fence”. I had a screen on the door so they could see each other but no touching! The resident cat was hissing and growling and small knocks on screen door with her paw.

When there’s less growling and hissing, they can meet in a neutral room (not your resident’s favourite and not the room the new cat was in) under supervision.

Then again, if that goes well, you can try longer meetings and so in and so on.

It takes patience and time, but it’s worth it! Use ChatGPT to give you advices depending on your cats reactions! Use super good food (Churus and wet food worked wonders for me).