A few years ago a breeder told me that when you get a new cat especially an adult and introduce them into a household where there is already a cat to keep the new cat in a separate room for a few days to a week with his or her own litter, food, and water. Go in to pet and get acquainted gradually. Then gradually open the door and let the kitty become acquainted with the rest of the house. This gives both cats time to get used to smells, noises, etc. without premature encounters which can lead to early territorial disputes. Another thing I found helpful was to let my older cat, Flory, smell the carrier I used for my new cat so that there wouldn't be quite the surprise later on. There were some squabbles at first when I let Jake out of the room but gradually they got used to each other, became friends, and seldom even hissed. I kept the food areas separate later on also.

Tried the same thing when I introduced Gracie to our household and it worked even better. We now have three cats who get along reasonably well. Two are really close buddies. The older one keeps her distance but never attacks nor fights with the other two. Good luck.