May the force be with you. I don't even like Star Wars, I don't know why I said that. May the odds be ever in your favor? That's better, but hopefully no one dies...
Step 1: Be crazy.
You need to be just the right amount of insane to come up with a books' worth of thoughts, write them down, read through them, and then berate yourself repeatedly until you questions your whole purpose in life...And then try and convince someone that it's the best thing since The Bible. If you're not crazy enough you will likely get a chapter or two into the book (one or two sittings) think "Well, that was fun..." and put it down forever. That's fine. It's what most people who start a book do, but you need to be prepared to push through that feeling. Write when you don't want to, even if it's just 500 words and it's junk that you'll inevitably throw out in a fit of rage. Eventually you'll get your groove back and you and Stella will have a finished book!

Step 2: Start with something.
You don't have to know exactly what is going to happen every step of the way but, good god man, start with something. As a freelance writer I wrote short stories, novels, non-fiction informative e-books, and I've started several of my own books and it feels like I do something different every time. I've started with no plan except one character (basically me in the flesh...or at least how I like to think of myself...) and a general idea of what I wanted the genre to be. That's right, the genre. I was pretty sure I wanted it to be Young Adult, but I didn't know if it would be fantasy, sci-fi, romance, etc. And then I really liked typing with a potty mouth so I almost threw the Young Adult out the window. Trust me. It doesn't work. Have a plan.