Face to face is the best way. It might be trickier than texting or writing a note, but in my opinion it seems more sincere. And if I were you, I'd built up to the part about cutting. Don't outright say "I cut" then walk away, it's best to say something like "I've been depressed for a while because (blah blah blah, etc etc), and I've been doing something that I'm trying to stop." And then you can tell them. Explain why. And I assume that you're telling them because you'd like help recovering? So at the end, ask for their help.
Good luck, and I hope this helps
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you're really brave for doing this. Let me know how it goes.