Well, first thing is that you need to show that you're concerned about him/her/other and then tell someone you can trust, such as a parent of yours and also inform his/her/other's parents, they might not be aware of the situation and in that point usually their parents seek help for them and then you just have to visit often, make sure they eat (for example invite them to your place and then make food) and that they won't be very lonely.

I have a friend who used to be very very very depressed, but his best friend pushed him to go places, meet friends, go to school and therapy and organized parties etc and ALWAYS invited him. They have had so many sleepovers and everything and she has really showed that she cares. I was so happy when this week he invited us to the beach for a picnic! And it's all because his best friend took care of him.

You talk to them. You figure out why they're self-harming and asks them what you can do to help. There are plenty of good advice but none of it matters if you don't talk to the person self-harming. So that's what you do first. And then you keep talking to them. You listen to them when they tell you something new and support them in what they're struggling with. And then you accept that it's difficult.

I think firstly anyone with the tendecy of self harm should received help and moral support from people around. Second thing is to build trust. You need to build trust so that they can share with you their problem. Third things is never judged and be as transparent as you can. Its just my opinion. Besically i think they need alot of support , space and time to heal at their own pace. For for severe case , i think its best to seek for professional help . Maybe you can try help them find a less harmful alternative for them to relieved themselves. For example instead of cutting maybe you can ask them to rub ice instead ? I read it somewhere but i forget . Also they can try healthy way like doing sports to release their stress like boxing , run , playing basketball . Last but not least they can hit on pillow or scream .. That would be less harmful. I hope it helps. This are basically my opinion.

Is that a close friend? Well, I had that situation too. It's really hard to get them to stop it, but all I could do was to talk to them. I bugged my friends for days straight just so she can't have time to think of suicidal thoughts. As a friend, all I could do was to make her feel loved. That's the reason why they do it, right? Right? They don't feel loved enough. They harm themselves, trying if they can still feel the pain. There's no way we can shoulder their pain, but believe me, the best thing you could do is to be there for that someone.
Try to have that someone open up to you. Maybe they can't express themselves, they're keeping their pain to themselves. Opening up may not erase all the pain, but I'm vertainly sure it would make it a little, little, little less heavy. If that someone doesn't want to open up, what should you do?
Open up to that person. They can't open up maybe because they can't trust you, they don't know if you can at least, at least, handle what they're feeling. Open up to that person first, make that person feel as if you're trusting him, and then they would trust you too.
Just be there for that person. Love is all they need.