All writing begins with a first draft. For some this is the hardest part. Have no fear. This is a private matter between you and your computer. No one has to see it if you want it that way. Yes, I assume you have a computer. It makes producing a finished product manifest much faster. If you write pen, or pencil, to paper that also works well. Nevertheless, the objective is to write. Some writers need an outline. I’ll discuss this later. For now, write; especially if you know your story. Simply, write. Do not bother checking spelling. Write as fast as you can. There will be time enough to return to your work and rewrite, edit, rewrite, edit.