The Code of Conduct Handbook written by Ola Sendecka and Ola Sitarska is a good resource on this topic.
How to take a report on code of conduct-related issues
It’s important to maintain the privacy of the reporter and all others involved during this process, so take the report to a private communication method or space as quickly as possible. Decide whether you’ll take reports via email, phone calls, other message systems, or in person. Agree with your fellow reporters and response team on what kinds of notes you’ll keep and how they’ll be stored. You may need to consider trade-offs between privacy, security, and accountability, as well as any legal implications for the information you gather.
Never contradict what the person making the report says to you, even if it doesn’t make sense at the time or you have different opinions about the people involved. Ask clarifing questions if needed to better understand the situation, but be aware that making a incident report can be stressful or scary. Offer the person a quiet place to recover afterward, if the report is made in person.