Choose an isolation room – a bathroom or bedroom. This room should have all the necessities (litter box, food bowl, water bowl, bed, toys, etc) that the new kitty will need. The kitties should be able to smell and hear each other from their opposite rooms. You can start feeding the kitties on the opposite sides of the doors. This will help associate the smell and sound of each other with a positive experience.
After three or four days, switch the kitties’ locations (switching rooms) so they can get more acquainted and used to each other’s smell.
After a few more days, start playing with each of the kitties near the door, helping build up more positive associations with the scents of each other.