The best introduction is a slow one. A lot of it will be guided by their reactions but the general rule of thumb is as follows:
- Start with your existing cat in your home as normal, and give the new addition his/her own room away from your cat. Swap their bedding around so they get used to one another’s scent. Be sure to give your cat as much attention as she normally has - throughout the entire introduction process, never let her get less attention than she is used to already!
- After a day, leave the door to newbie's room ajar by about 1cm. This way they can just see each other and can get used to each other’s smell.
- After a day or two, open the door to be about 2 inches open – just enough that neither can squeeze through, but get a paw through. They can get lots of hissing and swiping out of the way during this process and really suss each other out.
- Once they are calmer about these interactions through the door you can start supervised interactions - not for too long at first unless it goes REALLY well. Don't stress about hissing/swiping etc., they need to sort things out themselves and establish who is boss.
- As things get better you can increase time together til you're at a point you don't need to be watching them all the time. Until you are really confident things aren't going to escalate to a full cat fight, don't leave them together when no one is home, but generally it takes between 7 and 10 days for things to be pretty comfortable!