Give it time. That's what we're doing.
We're having issues with a new kitty (now 14 weeks old) and an 11-y/o kitty. 11 y/o kitty was hiding and observing new kitty from a distance for the first week. Starting in the second week, old kitty violently yells at and chases new kitten all the way across the house. She is showing no signs of slowing this aggression down as we enter week 3. We are trying to give older kitty as much attention as possible while hoping for the best.