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.
Besides, your friends may be 100% behind you in your new venture, but, in case you are hoping to involve others who have actual money to invest, you may need to be able to make a convincing case. Wouldn't it be nice to have anticipated possible questions and be ready with plausible answers? If you are risking your own money, that is perhaps even a stronger reason to do some indispensable planning.
Apart from this, they also analyze consumer trends and gather legislation reports and regulations that might have a future impact on your particular venture. These expert individuals make it their responsibility to build upon previous experiences and the requirements needed for you to carry out your intended plan.
There are a number of ways of going about the process of writing a good business plan. One of the most tried and time tested ways that works well is using a business plan template.