There are a number of ways of going about the process of writing a good business plan. One of the most tried and time tested ways that works well is using a business plan template.
Executive Summary - One to three pages that summarize all the information to follow; come back and write this last.
Appendix - Resumes of principals/management; letters of recommendation from current business associates/customers/suppliers; marketing research data; demographic data; leases or contracts in place or as promised; business licenses; price lists from suppliers; trade or industry articles or data; floor plans; information on subcontractors; liability insurance policies.
Marketing - How is the product or service priced? How will it be distributed? How will it be promoted? Will it be promoted by the venture or an outside agency? What agency? How have you determined what amount to set aside for marketing? How have you determined product or service forecasts? Possible Data Sources: on-line searches; Amazon; local outlets; trade journals; industry attorneys & accountants; salespeople.