If she has memories of you being a tomboy, you might start by reliving some of those to set the stage, and lead into "As you can see (indicating your male attire), I never outgrew that. So I'm seeing a therapist, and they agree that I have Gender Dysphoria. Mom and Dad and all my friends call me Nathan now, and I live as a guy."
If it was my grandma, I'd make it clear that I don't expect her to understand, I don't expect her to call me Nathan or "he," I just wanted her to know, so when people refer to me as Nathan and him she will know why.