Patience. Gradual introduction is the key. Put the cat in a room with litterbox, food, water and scratching post. Go in and get down on her lever and quietly whisper to her. Take a towel and rub down your resident kitties. Let the new cat smell it, so she gets used to her scent. Do the same things with the new kitty. Rub her down with a towel and let the new kitties sniff it to get them use to the scent.

Get down on her level and toss treats her way, gently talking to her. It's gonna take time and patience. But in the end, it'll work out. Just give it time.