When bringing a new kitten home to your older cat, plan for a gradual introduction.
First, let your kitten explore her previously appointed private space while your older cat gets used to the new smells. After a few days, it’s the kitten’s turn to check out the house while your senior cat retreats to his own space.
Then, let the cats see each other through a screen or gate. Once they start sniffing noses or rubbing against the surface, let them meet face to face. As they become more comfortable, you can increase their supervised time together.
Some hissing or swatting is normal as your resident cat sets boundaries. Try playing with both cats together, praising and rewarding them — especially your senior cat — if they’re tolerating each other.
This process can be tweaked — just keep gauging each cat’s stress level along the way.