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. If you know all of these except for Marketing, say, then this is where you will need to invest some time and effort. You can find a wealth of information by utilizing the traditional data sources such as chambers of commerce, major cities' websites, trade associations, the US Census Bureau, trade journals, magazine and online articles and advertising, etc.
Another point to bear in mind is that you need to keep the business plan relatively simple. Don't use fancy business jargon or clichés, since this will only cloud the important issues. In particular, if you are presenting the plan to a vendor or bank manager, you will need to keep the plan on point and free of over-fussy language and business speak. Present the facts in a concise, straightforward manner, and this will result in a plan that's both accessible and plausible.