Most people assume that if they don't need a loan to open their business that they don’t need to write a business plan, but every business should have one. It will help you figure out many key business elements, but most importantly, it will force you to look at your idea realistically and objectively.
When you’re done, you will have a better idea of the effort it will take and whether it's a venture you want to pursue at this time. Don't be scared. The process of writing out a business plan could spark your creativity and lead you to new ideas about this business and potentially even new ones. You'll also find that having your business goals written down enables you to refer to them at any time. This will ensure you don't lose sight of your original focus once you do start your business.
A word of caution, listen to your gut. As you write a business plan, it's tempting to paint the best-case scenario. I can get my supplies for this much, I can sell my product for this much more and I can sell a million units in the first 12 months.
You will know when you are not being honest because you will feel it in your belly. Listen and be honest, it's you you're talking to.