Diane Tarshis has been writing business plans for clients for over 10 years. Here she shares the tricks of her trade.

MORE: At what stage should you start thinking about your business plan?

DT: When you’re looking for funding, whether from a bank, venture capital firm, angel investors, or family/friends—the first thing they’ll typically ask for is a copy of your business plan. A business plan is your blueprint that lays out your idea, your premise and how and why your new venture will be successful.

Most people don’t write a business plan unless they’re forced to, and they’re forced when they need to raise money. But it’s important regardless of whether you need to raise funds. Writing a business plan enables you to identify challenges and missteps before they impact your business. It’s better to avoid obstacles through good planning, rather than waste valuable time, energy and resources to fix them down the road.

MORE: What are the basics of a business plan?

DT: A business plan is  a road map, what you’re going to do, and how you’re going to do it—profitably. A business plan consists of two parts: the words and the numbers. I help my clients tell a coherent story, and prepare the numbers to support that story. Another way to think of it is that a business plan needs to answer three large questions: What is the problem/opportunity? What is your solution? Why will customers pay for your solution? If customers won’t pay for your solution then you don’t have a viable business idea.

Of course there are a number of specifics that need to be included in any business plan: a description of the products/services you’re selling, target markets, the competitive landscape, marketing/sales plan, the members of your management team and, of course, projected financials.

MORE: What if you think you’re bad at math? Can you still write a business plan?

DT: Definitely! Even if you can’t afford to hire a professional business plan writer, there are numerous free and low-cost resources available—whether it’s workshops, free counseling or books that you can buy online or in bookstores—that can help explain what spreadsheets are needed and how to put together strong financial projections yourself. Having said that, I always recommend that my clients have a good accountant and a good lawyer standing right by them.

MORE: How long should a business plan be?

DT: I would say 40-45 pages is reasonable. I can’t tell you how often investors have told me that they roll their eyes when they receive a 100-page business plan. That typically means the entrepreneur is unfocused and too long-winded, so the plan usually ends up in the trash can.

MORE: Where can you go for concrete examples of what a business plan should look like? Or other resources to consult?