Is there a trusted third party that can help you tell them? I knew nothing about SI until my daughter told me that she had been harming herself - at the time she had quit for about 5 months, so she was not active, but even then it was not easy to hear. I think it helped a lot because we had a meeting with her and her youth pastor.

As someone who completely did not understand, it took a while to comprehend that SI isn't about suicide at all. I'd bet your father might have that misconception, so I would advise that you start off explaining that.

Regardless, Chaotic's advice above is still sound.