Small business consultants are very much on top of their game. They compile industry reports relating to the business, conduct feasibility studies, as well as studies relating to potential customers.
You may find that the easiest part is the actual writing of the plan. The real work comes in the data-gathering, which may take you a hundred hours or more, depending on what you already know or have researched. If your new venture is in an area where you've been working, you may already know about your customers, your suppliers, your marketing plan, your organizational structure, your financial and cash flow needs, equipment, inventory, and so on.
But your idea may be so new that you may also need to talk to potential customers, host some focus groups, talk to an ad agency, or maybe even make a prototype and float it past some people. Be prepared to spend the time. Remember, it's not about the Plan but the Planning.
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.