Instructions
Create a separate space for your new kitten. Put her in a room away from your older cat where she can feel secure and where your older cat won't feel threatened by her presence. Provide your new kitten with her own water bowl, food bowl and litter box.
Allow the older cat to become familiar with the new kitten's scent. You can accomplish this by letting your older cat smell the new kitten's scent on your clothes, by exchanging their bedding and by allowing them to sniff each other under the door. The latter should be done after giving the cats a few days to become used to each other's scents.
Place the older cat's food dish on one side of the door and the new kitten's bowl on the opposite side. The Ohio State University College of Veterinary Medicine claims that this builds positive associations between food and the new kitten's scent.
Crack the door open so that the older can can see the new kitten. You should do this once the two cats have gotten used to eating on opposite sides of a door. Introducing a kitten can make your older cat aggressive, so don't allow them to have access to each other. Hissing is OK, but if they become too aggressive, shut the door and separate them until they've calmed down. Repeat this until the older cat no longer acts aggressive toward the new kitten.
Allow your older cat to briefly meet your new kitten. Do this by opening the door wider while you watch to be sure the cats don't fight. If the older cat becomes aggressive or the new kitten seems overly defensive, separate them until they calm. Repeat this until the two can interact without aggression.
How to Get an Older Cat to Accept a New Kitten
Adopting a kitten can be a fun and exciting event--but it can also mean trouble if you have an older cat who doesn't appreciate the new kitten's arrival. A cat who has spent his entire life in a one-cat household will likely have more problems adjusting than a cat who has been part of a multi-cat household. He may never fully accept a new kitten in his life, but with a little effort and patience, it is possible to get an older cat to tolerate the presence of a younger cat.