keep the kitties in seperate rooms for a while, one with a door between them so that they can become aware of the other one on the other side.
Eventually they will start playing with each other under the door, let them do this for a while and then one day, open the door and leave them alone to discover each other.
This may take a day or two or a couple of weeks.
It's worked well for me.
Incidentally you do not have to keep them in the rooms, you can allow one kitty out for awhile and then the other kitty, just always keep a door between them, until they start getting along.