Start with a detailed outline. The more details, the easier it will be to expand. Summarize each section of the novel as much as you can in the outline. Have major plot points, story arcs and fleshed-out characters worked out in the outline.
Separate the outline into a beginning, middle and end. Write a short summary of each section, no more than a few sentences for each section.
Break down each section into chapters. Start with a sentence or two to describe the events to unfold in each chapter. Then gradually expand these descriptions (go from two sentences to four, four to six, six to eight and so on).
Flesh out each chapter with the most important action, dialogue and descriptions. Reference any character history you might have written. Keep to the story arc and plot line.
Go back over the chapters and fill in less important details once you have added all the major elements. Expand on the descriptions and even add to dialogue, if needed.