What should I write in my fan mail?

Hi all, I am writing fan mail to someone in Japan. Their manager is expecting it since they replied back to me, and I told them I am sending. I am nervous and never sent fan mail before. Unsure what I should even write or say or talk about. I want to express to this person how much they mean to me in my life and what about their work I liked. >3<.

I would love some ideas because this person is not a huge famous person. So, If they do read the letter I want to make sure I say everything I want!