If it's really bad, I take the new cat and put them in the same room with the resident kitty, only the new cat is in a plastic cat carrier. I stay in the room and watch them. There is usually some hissing and such, but in this way, the resident cat does not perceive the new cat as a threat, since they are in a carrier. I do this for about 15 minutes to a half hour once or twice a day. This helps the old cat get used to the new one in a non-threatening atmosphere. The old cat gets used to the new one's scent and sounds. After a few days, the hostilities stop and curiosity takes its place. After about a week to ten days, I introduce them face-to-face and while there is ususally some hissing or checking each other out, this soon either leads to friendship or just them ignoring each other. I'm wishing you the best of luck! This method has always worked for me.