Letters, even when sent by return of post, always give you time to reconsider. The inbuilt delay of the postal network reintroduces patience to a correspondence. A letter always comes from the past, from elsewhere, rather than from the ever-insistent here and now of the digital. Thinking about writing a letter is different from thinking about writing an email. (McGregor, 2013).
Write something about yourself. No need to be fancy, just an overview.