You have to do two things. First whatever you are going to write just get it out on paper or a computer memory. Don't worry about grammar or spelling or even consistency that first dump is your passion, you can clean it up later, but if you don't get it out you may never do it. Second you have to treat it like a job, and find a way to write that allows you to make steady progress. For me it was writing an hour a day every day.