Start with free writing your ideas.
Start with free writing to get it all out of your system. Do not worry about the title, concept, plot, theme, characters, names, ages, settings, items, time, grammar, punctuation, or anything else that can stop your flow.
When you Free Write, you are letting your juices flow. Take the story that you have in mind and write it out. Watch how things will come out and form. New ideas and things that you did not think about will flow with each word.
By having this at the beginning of the book it gives you the chance to be more open to a greater story.