How do I tell my friend about this?

I have cut my arms before, and your right for considering whether to tell ur best friend or not. I'm not sure if I can be much of help because I haven't told anyone of my problems, but this might work. First get into the topic of what she already knows, and sorta get her opinion on it and stuff, then tell her some of the things she doesn't. If she totally freaks, then maybe u should stop, but if not keep talking. There's no doubt that it's going to be uncomfortable, bcuz what would you be feeling if you were in her situation?