The best way to report police brutality without putting yourself in danger is to wait to report until you are in a safe location. This means do not approach the offending officer during or immediately after an interaction. If you trust higher law enforcement officials in your area, you can call the police chief to report the incident.
It helps if you have details and are able to provide some evidence, but even if you don't have proof, you can still report the incident so it is on the record. This might help if another incident happens in the future. If you do not trust your local law enforcement chief, you can call the American Civil Liberties Union (ACLU).