Keep their territorial instincts in mind
If you’re bringing a new cat home, it may take a while for your existing cat to accept her. Cats are extremely territorial beings. Introducing them may take some time. So you will have to be very patient with them. Separate them physically at first, maybe with a screen door or a baby gate. Let them adjust to each other’s scent. Once they’re ready, allow them to have food together. This will slowly but surely have them be friends.
Take it slow
As a new cat enters your home, your older cat will feel threatened. That’s just their nature, so the introduction phase may take a little longer than you expect. Give the new cat some time to settle in. Then bring your older cat to catch a glimpse of her. Keep them in different rooms and allow meetings only from across the door. After a day or two, as the cats have adjusted to each other’s smell, slowly introduce them.
Be patient and keep trying
Territorial aggression is pretty common in cats. So introducing two cats may feel like a drag. But bear with them and you will soon find them hanging out on the porch or the windowsill. But remember, you can never introduce two cats right away. You will have to be very patient during the process. There may be some hissing and running, but never let them engage in fights. It is always a good idea to keep them separated for the first couple of days.
Socialize your puppy