How do I tell my mom I started cutting again?

I'll make it as easy as it gets.
Be honest with how you feel.
First, tell her how you feel about telling her this- its hard for you to say, and explain why with your feelings. Then, tell her what you have been feeling that has led you to cut, but don't actually tell her that you are cutting again until you get ALL of your feelings out.

Do it in front of her, she'll get the message.