We make our spare bedroom into a kitty apartment with litter, food, water, toys, soft beds, etc. We keep the new kitty in there for about a week and let the two play patty paws under the door. After about a week we let the new kitty loose in the house, but not outside. They won't immediately take to each other but the slow introduction slows down the hissing and paw swipes that never hit anything. After this, it is pretty much up to them. Don't interfere, it slows their adjustment. they have to work things out.