Slowly introduce the new cat's scent before full interaction. Let them gradually get used to each other.
Give new cat their own safe room with food, bedding, litter box. Let current cats inspect from outside.
Swap bedding between new and current cats so they get used to each other's smells.
Feed cats on opposite sides of a door so they associate each other with positive things.
After a few days, do short, supervised meet-and-greets until cats seem comfortable.
Introduce highest, then lowest ranking cat first. Save most territorial cat for last.
Have toys, treats ready to divert attention. Separate if signs of aggression appear.