For those of us who love cats, it is very common to have more than one. Cats can be very good friends with each other and great playmates, but introducing a new cat can take time and patience. Cats are generally found in two territorial positions: the resident cat dealing with the presence of an intruding cat and the visiting cat entering unknown territory where there may be danger. Both positions are stressful and can cause conflict between them if the introduction is not done correctly.
Experts recommend keeping them completely separate at first, especially designating a restricted area for the new member of the family equipped with a litter box, water, food, sleeping blanket and scratching post. This way the new cat will feel safe and comfortable upon arrival. After 2 or 3 days of being apart, it is recommended to exchange your locations but without seeing each other. The resident cat will move to the area where the new cat was and vice versa. In this way, each one will get used to the smell of the other in a non-invasive and gradual way. Some ethologists also recommend rubbing both cats with the same towel to mix their scents. Another option may be to rub a cat's face with a cloth or sock so that it soaks up its pheromones and then leave it where the other cat is.
After 3 or 4 days you can let them see each other for a short time, from a distance and always under your supervision. Gradually day by day you can let them get closer and interact. It is normal that at first one meows at the other, completely ignores each other, or hides. Little by little they will realize that one is not a threat to the other and they will be able to live together. If after following these steps there is still stress and fights between them, a pheromone spray or diffuser can help.