The correct procedure
Introduction should be by scent first.
1. Keep both cats in a separate room for at least 2 days. Your cat will know a new cat is in the house by its scent but will not be able to confront it directly. Swap beds between the cats daily
and rub the cats all over the face and flanks with the same cloth to swap scents. Scent is very important to cats and is the best way to start a bond.
2. after 2/3 days of separation place the kitten in a carrier, open the door and allow your cat to meet the new cat in her own time. Don’t open the carrier. Let them sniff and investigate through the bars for 5 mins or so. Both may show some aggression or fear but hopefully both will simply sniff each other at this stage.
3. let them keep meeting like this for a few mins at a time at least 6 times a day for a couple of days. Then begin by letting both cats interact outside of the carrier. Make sure you oversee the meeting and use caution. don’t shut the cats in one room when they meet as they both will need areas to run and retreat if they need too.
4. A little bit of hissing and wrestling is normal the first few weeks as both work out their place in the feline hierarchy of your home.
5. Give plenty of attention to both. Use separate food bowls and food stations. 2 litter trays. They will soon work out their territory between them.