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.
On the subject of finance, you need to produce realistic financial details. How much money will the business require? What are the expenses required? How much tax will you need to pay? What are the profit margins? Do you stand a good chance of achieving these sums, and how? Finance plans should include income statements, cash-flow statements, balance sheets and profit analysis. This should form a big part of your business plan, in order to convince the vendor or bank manager of your credibility.