There's several techniques to introduce cats.
1. Keep the new cat in ONE room and keep the door closed at all time. That cats will get used to each other through the door.
To further help..
2. Keep a blanket/towel where you new cat lays, and where your other cat lays.. After a while, swicth towels so they get used to eachothers scent/
And for a proper first introduction,
3. Keep the new cat inside a cat-carrier when introducing. They will hiss at first, but doing this correctly will help prevent future problems.
I also suggest keeping new kitty away while he heals from surgery.