How do you handle clients who keep changing project requirements?

Hi everyone,
I’m a Full-Stack Developer and I’ve worked with clients on web and application development projects. One challenge I’ve faced is that sometimes clients request additional features or make major changes after the project has already started.
How do experienced freelancers handle this situation? Do you define a fixed scope of work before starting, charge separately for additional changes, or use a formal change-request process?
I’d appreciate any advice on how to manage changing requirements professionally while maintaining a good relationship with the client.
Thanks!