Compartmentalizing your plan by developing planning modules allows you to attack the plan in parts, yet still maintain a cohesive plan. I have found that developing an annual plan made up of quarterly targets - thus becoming a rolling quarterly forecast financial model - allows for a cohesive structure along with the nimbleness to react to market conditions. At the end of each quarter, a true-up process to align results to annual targets needs to be re-forecast and adjustments made.
The first step involved in writing a business plan is the executive summary. Here, include everything that you would cover in a five minute interview. Explain the fundamentals of the proposed business: What will your product be? Who will your customers be? Who are the owners? What do you think the future holds for your business and your industry? Make it enthusiastic, professional, complete, and concise.