Anybody can say they're an expert, but until you back it up with some evidence no one's going to believe you. Today I visited a blog that told me in no uncertain terms this person was an expert – but it never said why or how he was an expert. Do you know what? I don't believe he is – would you? You don't need every detail of how you came by your knowledge – just give enough background so people can relax and have some confidence in what you write, what you say and what you sell.
Did you learn your expertise in school? By trial and error? By practice? By interviewing experts and learning how they do what they do?
Then tell your story.