Start with separation
Give the newcomer their own room and essential resources. Let each cat feel secure before beginning introductions. Swap familiar scent items without forcing contact.
Make progress gradual
Move from scent to carefully controlled visual contact, then short supervised meetings when both cats are relaxed. Return to an earlier step if either cat becomes distressed. There is no guaranteed timetable or promise that cats will become friends.
Keep resources separate
Provide several resting spots, feeding areas and litter locations so one cat cannot monopolise access. Keep individual notes under each cat’s profile. Seek veterinary or qualified behaviour support for persistent conflict.