This is a really nice guide that they’ve put together:
“This booklet aims to help you understand more about self-harm and what to do if you are worried about yourself or someone else. It explains what self-harm is, what to do if you or someone you know is self-harming, and how to get help.”
The help resources are UK based, but even for those of us not in the UK, this book is absolutely chock-full of educational material around the issue of self-harm. You could do worse than check it out if you, or someone you know, is dealing with it.
https://www.mentalhealth.org.uk/publications/truth-about-self-harm