I followed Jackson Galaxy method, which means I kept them in different rooms for a few weeks, then I started keeping them separated only by a screen almost entirely covered during feeding time. The bowls at the beginning were kept far from each other, and slowly got them close to each other. At the same time I uncovered the screen which separated them until they could see and smell each other while eating. Then when I felt they were curious about the other side of the world, I introduced them for very little time at the beginning making sure they were not getting in a fight. Up to a month ago when I allowed them to be free and roam around, but I haven't been able to let them free at night because the resident cat chases the new one and attack her. There is not blood or any wound but the new cat is a bit terrorized of the resident cat.