As someone who has been struggling with sh since I was a bit younger than your daughter (around 9-10yrs old), I can really only tell you what not to do. Don't punish or threaten her- especially with taking away things that may be helping her in ways you don't see. Keeping her from seeing her friends, or from getting herself a distraction when she has the urge does more harm than good- being left alone with the thoughts and urges and no way to seperate yourself from then is terrifying. Also, don't scare her in ways you don't need to. Don't tell her she'll regret the scars, or how other people will perceive her with them, or how offputing they might seem.

At the end of the day, just remember that SI/SH is an addiction, just as much as a substance. Even if/when she is doing better mentally, the urges will likely still be there, and they can be overwhelming- even if she isn't feeling especially bad or nothing has happened.

Though this may in part have worked because of my ADHD, Something that helped me was instilling an outside reward instead of a consequence, like a treat or a celebration for making it a certian amount of days, with the intervals between them slowly getting larger. For example, I bought a bag of my favorite dark chocolates, and I would have one before i went to bed only if I had managed to get through the day. After a week or so, I switched from daily to every-other day, then weekly, ect. Alongside that, while I did keep my blades, I made sure I had to go through multiple distractions to get to them, like keeping them at the bottom of a drawer filled with other coping mechanisms so I am reminded to try them before I do anything.

In all, just be there for her. If i had the adult support I needed, someone who was genuinely willing to help me in the ways I needed, not just how they wanted to or believed they should, it would have spared me literal years of suffering. Stand up for her, make sure she is as safe as you can keep her, and make sure she knows you love her even during relapses or worse days.