You must begin by making notes. You may have to make notes for years…. When you think of something, when you recall something, put it where it belongs. Put it down when you think of it. You may never recapture it quite as vividly the second time.
2. Make a detailed outline of your story
Invent a system Zolaesque…but buy a file. On the first page of the file put down an outline of a novel of your times enormous in scale (don’t worry, it will contract by itself) and work on the plan for two months. Take the central point of the file as your big climax and follow your plan backward and forward from that for another three months. Then draw up something as complicated as a continuity from what you have and set yourself a schedule.