Like others have said here, go VERY SLOWLY. Keep them completely separated for the first week at least. They will get the idea that there is another cat in the house which is stressful enough. The swapping towels works extremely well; gets them used to each other's scent. Introduce them very slowly to each other - supervised visits of about 10-20 minutes a day after about two weeks of separation. You will want to rush and think the cats want to meet each other - they don't. It's very stressful for both cats so go slow. They will love you even more for it.

When you do introduce them, make sure to give them each equal time, pets and affection in front of the other cat. That way, hopefully, one won't get jealous of the other getting more attention.

It took about a month before I could leave Mitzi and Mishi alone together. After two months of gently and slowly introducing them to each other, they're FINALLY playing together, eating each other's food, he's licking her, she's starting to lick him, they can even sleep on the same bed at the same time without hissing, although they're not cuddling (yet). Taking it slowly does pay off in the end. Your cats will adjust to each other and let you live a peaceful and cat-loved life. Good luck!