I myself wrote them an overly detailed letter explaining how I felt, stats, psychology, requests for changes (e.g., pronouns, refer to me and sibling as "kids" and not "girls"), and left it on their bed around 12 @ night. Not everyone will be able to do that. I was lucky enough to 1. still live with my parents (I was 12 1/2 when I came first out) and 2. Have welcoming and supportive parents. Albeit a little oblivious at times. If you feel safe and ready to tell your parents, go for it, but don't force it if you aren't ready.