Introducing cats to one another is a little different to introducing a cat to a dog. Cats are not naturally good at interacting with cats they did not grow up with. It is extremely important to not rush this interaction and to understand that the interaction can take anywhere from a few days to a few weeks.
Always remember the 'plus one' rule! It's possible for the cats to get stressed if they are forced to share food/water bowls and litter trays so always have one per cat and then one extra.
Start by having the new cat in their own safe space and give them a blanket. After a little while, allow the current feline occupant to smell some of the new cat's belongings before they are physically introduced and let the new cat smell some of the old cat's belongings. This will allow them to recognise each other's scent when meeting for the first time.
For the first interaction, let them see each other but not come into physical contact. For example, this can be done through a door with a glass panel or a crate. Once you've done this a few times, you can then allow physical interaction. Allow the cats to come to each other in their own time and don’t force the interaction. Be present at each interaction until you can comfortably leave them alone together.
Remember, there is no rush to this process so let it take as long as needed. As discussed above, pheromone diffusers can be extremely useful in these situations and some are specifically designed for multi-cat households, like “Feliway-Friends”.