your pets. that is, use a toy that will go under the door. let both your old and new pets play with it at once. that will introduce new smells and get them to (hopefully) play together.
trade pet scents. take the bedding of one cat or pet, and switch it out with your new kitten. that way, they can continue to smell each other. you can also just gently rub a towel on each pet, then place it where the other pet can smell it.
limit access. it's best to only allow your cat to explore one new room at a time. if possible, only give her access to one new 