How do I deal with friends who self-harm?

They're still human. The best thing to do is just be there and let them know they are loved. Self harm can be triggered by multiple things that vary with every person, so if they know what their triggers are, try to avoid those.

Treat your friends who self-harm the way you would want to be treated if you were in emotional pain. Be gentle and kind, listen and respond carefully. Be attentive to their realistic needs; whether or not they need a higher level of care, this is important to discuss only if you are close enough with the person and they feel safe confiding in you. If you see self-injurious behaviors, talk to your friend and ask what is going on and what, if anything, you can do to help. Grab them water, bandages, whatever they may need, and stay by their side until the nightmare is over.

Well, for one, you don’t “deal” with them. You show them support and empathy and though you may not understand what they are feeling, you can always be a shoulder to lean on. Check up on them and ask them how they are feeling and if they’ve tried alternative coping mechanisms today.

Support them, that's the biggest thing. They need you more than you know. Have an open and honest conversation with them. Tell them how much they mean to you, and that you love and support them no matter what. Help them find healthy coping mechanisms, and hold them accountable. Self-harm is extremely addictive, and it needs to be treated as an addiction. They may not even realize how dependent on it they are. As their friend, you need to reach out and help them see that it is unhealthy. Be patient and gentle with them, they are probably in a pretty fragile state as it is. They turned to self-harm because they needed something, they were hurting and struggling and they didn't know what else to do. Support your friends, or you could lose them. That is the risk they are dealing with, and they need you in their corner.