Write it all down in a notebook. It doesn't hafta be the actual story, but a synopsis of what each chapter, or whatever important points are about. Doesn't even need to be in order, just get it all out on paper, so it can be sorted out when it's time to actually write it. That's what I do, although for a different reason, I do it when I'm already writing too many things and this new thing won't give me peace to work on what I already am, so I write it down to get it out of my head and not forget it. It also lets you see how viable it is to actually write. I had an idea once, that I wrote down and after reading it well later, found it just wasn't going to work, like I thought when fantasizing about it.