When you find your newcomer, ask if you ca brush it. Collect the fur and take it to your cat. Let the cat smell it. If they react with curiosity, rub on it drool on it, I think it's a good sign. Be careful if they hiss or spit or go crazy over it.

Then follow all the things that Wayne said.

When you've got your newbie, do an 10 -14 day isolation. (no kidding)
For health sake as well as getting them accustomed to their environment, then put your cat away briefly and let the new cat out in the resident's territory. Do this a couple afternoons, you'll know when they are ready to see each other.

If you follow integration procedures, you will most likely have happy cats. There are some though that strictly need to be only cats, and no one can predict this well, only be trial and error.

Good Luck.