We have done this at least 4 times in the past 3 years and have had good results.
Step one, is to be patient. Don't get too involved in the process as you will only let your own anxieties influence the cats.
Step two, introduce them slowly, try and have an area that's neutral and provide them with their own food and water areas in different places.
Step three, most vets sell sprays that have calming effects on cats that do not harm them. I have only used it once, and it seemed to help a little, but my guess is that the same result, (them doing fine) would have happened on it's own.
Cats are territorial and have their own comfort zones, when a new pet is introduced there is a "re-zoning" that takes place over several days. We have experienced a lot of hissing, a few swats, and one or two short bursts of aggression. (at this point we use spray bottles with water and separate them for a time out). But eventually as they get used to each other and their smells they adjust. They may never become "best friends" but we have been surprised more than once when several weeks later on a cool day to see them sleeping next to each other.
Just make sure they have their "own" space and an exit to retreat to when they want.