How do I come out to my mom as a trans gender?

Before explicitly using the word transgender, you can prepare her for what you have to say by slowly telling her the story how you discovered it, what feelings made you question yourself, what felt good and what didn't. Then you can tell her about any worries or doubts you might have had, and how you finally settled that this is who you are. Make sure she knows you're open to her questions, if there's something she doesn't understand. Let her know you told her because you love her and you want her to know the real you. Best wishes!