When you're writing a business plan in a situation like these, you need to address a few issues the intentional entrepreneur has already pondered. The first is do you really want this idea to become a full_blown business? Certainly it's flattering when you realize there's a market value for something you were doing anyway, but that doesn't always mean you should launch a business. A lot of accidental businesses form around fads or seasonal items, and may not be robust enough to function as year_round, money_making, enterprises.
Marketing Analysis/Strategy: The next thing to write (after the general description) should be your marketing strategy. For new or existing businesses, market analysis is an important basis for the marketing plan and will help justify the sales forecast. Existing businesses will rely heavily on past performance as an indicator of the future. New businesses have a greater challenge _ they will rely more on market research using libraries, trade associations, government statistics, surveys, competitor observations, etc. In all cases, make sure your market analysis is relevant to establishing the viability of your new business and the reasonableness of the sales forecast.