One room, fully equipped, before anything else
A foster cat arrives having lost every familiar landmark she had, so her first need is a small world she can completely learn: one quiet room with a hiding place she can reach without crossing open floor, a bed, litter tray placed away from her food and water, a scratching surface, and something soft carrying a calm household smell. Set it up before she arrives, sit with her in short, undemanding visits, and let her eat, use the tray, and start leaving her scent on the furniture. Those ordinary milestones, not bravery around people, say she is ready for the next stage.
Keep her physically separate from resident pets at first even if everyone seems relaxed, and wash hands between handling her and your own animals during the early days. Separation protects health as well as feelings, and it gives your resident cat or dog the same gift: time to register a new smell in the house without meeting its owner face to face.
Protect your resident pets' routines first
Introductions fail most often at the resident animal's end of the deal. A resident cat who loses her favourite room to the newcomer, finds her feeding spot moved, or gets less of her usual evening play reads the foster as the cause of a worse life, and that association is hard to undo. Keep every resident routine exactly where and when it was, give extra reassurance through the normal rituals rather than frantic new treats, and make sure the foster room is not the resident cat's most treasured territory if you can possibly avoid it.
Watch your residents for their version of stress: a cat eating less, over-grooming, or guarding doorways, or a dog fixating on the foster room door and unable to settle. Those signs are information to slow down, not problems to push through, because a foster placement that unravels a resident pet's confidence helps neither animal.
Scent first, sight second, contact last
Begin with scent, the channel cats trust most. Swap bedding between the foster and resident cat after a couple of days, or rub a soft cloth along each cat's cheeks and leave it near the other's food bowl, and watch the reactions: sniffing and ignoring is excellent, hissing at a blanket means the live version is far too soon. Feeding both cats on opposite sides of the closed foster door, at a distance where both eat relaxed, builds the association that the stranger's smell predicts dinner.
Move to sight only when door-line meals are calm for several days. A baby gate or a door opened a crack and secured lets them see each other briefly while a meal or play distracts both ends. Short and boring is the goal; end sessions while nothing has happened. Then short supervised time in the same room, with an escape route for the foster and your dog on a lead if a dog is involved, growing longer only as both sides choose relaxation over vigilance.
Expect setbacks and keep the timeline honest
A hiss, a swat through the gate, or one bad evening after three good ones is a step back to the previous stage for a few days, not a verdict on the placement. Foster cats carry unknown histories, and some need weeks at the scent stage before faces are a good idea. Rushing because the arrangement is temporary is the classic mistake; a slow introduction that ends in peaceful coexistence serves a foster cat's future adoption far better than a fast one that produces a fight story.
Keep a short daily note of the stage reached, how each animal ate and rested, and any flashpoints, so progress is judged on evidence. Sharing that stage plan and the foster cat's feeding and litter notes in KinsLe's Family Sharing stops one family member letting her roam free while another is still carefully feeding through the door.