Concretize means to make something concrete rather than abstract. Concrete in this case meaning 'existing in material form' not a mix of sand and cement.
Actually getting on and writing your story, as opposed to just day dreaming about seeing it in a bookshop, will concretize your brilliant idea into something you can submit for publication.
Concretize may also be spelled as concretise.
(This is an old photo of the booksigning for my first ever published book. It's now available in a newer edition.)