The first step is to identify the bully. This may seem obvious, but it's important to be clear about which cat is the aggressor and which cat is the victim.
If the bullying is severe, you may need to separate the cats for a period of time. This will give them a chance to calm down and learn to get along better.
Make sure that both cats have their own food and water bowls, litter boxes, and scratching posts. This will help to reduce competition and conflict between them.
Playing with your cats can help to reduce boredom and aggression. Make sure to play with both cats separately, so that they don't feel like they're competing for your attention.
If you've recently adopted a new cat, it's important to introduce them to your existing cat slowly. This will help to prevent bullying and aggression.
If you're unable to stop the bullying on your own, it's important to get professional help.
It may take time to stop cat bullying. Be patient and consistent with your efforts, and eventually the cats will learn to get along better.