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.
Here is a recap of where we are in this discussion: Business plans formalize an understanding of the task with appropriate analysis leading to a plan_of_action. Not all business plans are for profit motives. Business plans are for an enterprise effort and not focused on disciplines/departments, e.g. Marketing plan, sales plan, HR plan, supply chain plan, etc. Business plans are a great vehicle to build a team effort. Plans can be utilized for formalizing metrics relative to achieving goals and performance measurements. Some complex plans might include a Project Management professional. There are internal and external audiences for business plans. Most external focused plans are for outside funding of projects. Be mindful of the 'tone' the plan projects to the reader. Tone refers to the impression a person gets from reading the plan; a subliminal feeling about the plan.