How can i tell my mom about my self harm?

Seek help first: I would say that you should seek professional help for self-injurious behaviors. It is your decision whether you want to inform your mom or any other family or friends, but you need to get treatment first. If you don't already see a therapist or psychiatrist, ask for a referral from your primary care provider.

Straight forward: The most important part is telling somebody so you can get the help you need. In your own language, say it how ever you can just to let her know what's going on. In a letter. Text her. Show her. Anything that will get the ball rolling on support and healing.