As a very first step we are advising to contact the infringer if he or she can be identified and you have the ability to reach out to them. It might be the most efficient way to tackle the problem, as some people do not feel well about their wrongdoing or do not want to have any trouble. This option is especially effective if the platform or domain is well-known and they to do not want to lose their reputation or have trouble with admins.
The recommended message for contacting the infringer directly (could be less formal and different, but we are advising to use similar structure and logic):
Copyright infringement notice
I became aware of the content you have published on the following link:
I am the original author of this material and I own all Intellectual Property rights to it. Since the content was published without my consent or knowledge, it constitutes copyright infringement.
Please remove this content as soon as possible. Failure to remove the illegally published material will result in further escalation steps from my side. If you will not comply with my legitimate request, I might also consider taking legal action.
If you require additional information or wish to further discuss this issue, please contact me via email.
(first name, last name)