Telling a friend you self-harm?

I've decided that I want to tell my closest friend about my self harm, but how do I tell her? How do you think she will react? Any personal experiences? Thanks P.S. I'm not telling her for attention, I have been self harming for 3 years and haven't told a soul, but I think it's time that I need help for her and she has self harmed in the past, but hasn't in a few years.