Coming out to my best friend. I'm really anxious. Any advice?

So, I feel like it's about time I came out as trans to my best friend. I have a feeling she might already know, but I obviously have to tell her anyway. I'm just scared about so many things.

She's been my best friend since 8th grade, so about 3 or 4 years now. I think she would accept me, but I'm scared that I wont be her best friend anymore if she know I really feel like a guy, even though one of her other best friends is a cis male. I know that's crazy. I just dont want to ruin our friendship or ruin how she thinks of me. I know she loves me, but I'm just nervous that she wont like me as much seeing me as a guy. But I have to come out to her. I'm ready for her to know, I'm just anxious about the outcome. She'll most likely be okay with it, she's a very accepting person, but my brain keeps thinking of the worst case scenerio.

I'm going to tell her, the above was mostly venting, but I just need some advice. I have no idea how to even start a conversation like that. And how do I tell her that I'm trans? I'm not sure how to put it into words. If anyone had any advice, I could really use it right now..
Thanks.