Start by writing a description of your business, including what stage of development it is currently in (conception, start_up, first year, mature, exit) and your plans for growth. Discuss the nature of your business, the main products and services you offer, the market for your products and services, and how and by whom the business is operated.
I suggest that you construct easy to read tables and graphs for the financial portion of the plan. The worksheets suggested are: Salaries/Wages and Benefits, Outside Services, Insurance, Advertising Budget, Occupancy Expense, Sales Forecasts, Cost of Projected Product Units, Fixed Assets, Growth (or Start_Up) Expenses, and Miscellaneous Expenses. You may find some of the worksheet templates at PlanWare.org to be useful.
Here I shall be discussing the basic steps involved in writing a business plan: Executive Summary: 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.
If an online business, you need to go into detail how you will attract customers to your website. General statements like "I will use Face Book ads and email marketing" will contribute almost nothing to helping your cause unless you have detailed statistical analysis of tests you have conducted or of another similar business you have been associated with. If you do not have any data upon which you reference your estimates, it could show lack of proper thought to the remainder of your business plan.