Same as if it were a kitten. A cat is a cat no matter what age, to the other cat.
I had a cat, Gabby had been the only one for 12 years(RIP, baby girl, I lost her last year at 17), and my now hubby has two cats (7 years old).
We brought them to the house, introduced them with us present and let the howling and hissing begin. We let Gabby stay in our room for the most part (did not force her, it was her choice) and let the other cats get a feel for the house. Of course, we brought in another litter box because I did not want Gabby to reject hers.
They will fight, hiss, and act out but over time they will learn to get along or LOL, learn to get over it.
Good luck!