How do I introduce two cats?

Slow. The standard protocol takes 1 to 4 weeks depending on the cats. Start with the new cat in a completely separate room with her own food, water, litter box and resting spots, no visual access to the resident cat. After a few days, do scent swap (rub each cat with a soft cloth, give it to the other). After another few days, brief visual contact through a baby gate or cracked door. Then short supervised together-times that gradually lengthen. Cornell flags multi-cat household conflict as one of the most common behavior problems and traces a lot of it back to introduction speed.

When to call a vet: if either cat is hiding, refusing food, or showing repeated aggression beyond brief hisses, slow down. Persistent stress signs deserve a behavior consultation.