QUICK ANSWER
Document specific incidents first, check your employer’s complaint policy, go to HR or another named channel in writing, stick to facts, and keep records of how you are treated afterwards. If HR is not an option, the policy or an outside agency may offer another route.
Where to take it
HR is the usual route. If the manager is senior or close to HR, your policy may name another contact (a different HR leader, legal or compliance, an ethics line, or the manager’s manager). A union representative can help if you have one.
If you and coworkers are raising the same concern together about working conditions, the NLRB explains that acting together for “mutual aid or protection” is protected under Section 7 of the National Labor Relations Act, and that this can include an employee bringing a group complaint to management. Coverage has limits, so check the NLRB page for how it applies to you.
If you work in a very small business without HR, the owner may be the only internal option, which is why outside agencies and legal aid matter more.
How to describe it
Describe actions, not character. “He said X on this date” is stronger than “he is abusive”. Say what you are asking for, such as a meeting, a change in how decisions are communicated, or a written response.
“Since May 6, when I told my manager, Jordan Sample, that I had missed my meal break two days in a row, my schedule changed from five days a week to two without notice (May 9). On May 12 Jordan said, ‘If you want hours, stop complaining.’ I would like my hours reviewed and a written response.”
Names and events are made up for illustration.