"When you bring the new cat home, do so in a cat carrier," Hollow says. "When the carrier is empty, let the established cat explore it." Ignore any signs of hissing or attacking the empty carrier. Remain calm and relaxed.
Pellicano suggests reassuring your long-time cats by playing with them and giving them positive reinforcement with the new toys and tasty, vet-approved food.
Never leave a newer cat alone with an existing one during the introductory period. Most importantly, don’t change your routine.
“Cats are excellent at picking up nervousness," Hollow says. "Before you pet your present cat, wash your hands to get the new cat scent off your hands. Let each cat know how much you love them. When the hissing stops from separate sides, supervise an introduction.”
It takes several weeks for cats to acclimate to new surroundings, so have a separate space for the new cat to live for at least a week, Pellicano says. “Observe both cats. If there’s less growling and hissing and if neither cat goes off his litter box or loses his appetite, you can likely allow them to meet. Be sure you are present to observe for a day or two.”
Source : https://pets.webmd.com/cats/features/adding-another-cat-to-your-home