If an online business, you need to go into detail how you will attract customers to your website. General statements like "I will use Face Book ads and email marketing" will contribute almost nothing to helping your cause unless you have detailed statistical analysis of tests you have conducted or of another similar business you have been associated with. If you do not have any data upon which you reference your estimates, it could show lack of proper thought to the remainder of your business plan.
For an online business, a detailed and in depth business plan is usually not necessary unless you are trying to combine your online business with a traditional business. For most online business startups, the detail involved with planning a traditional business is not required. However, it would still be beneficial to you if most of the topics were still covered, even if only briefly. Having a written plan in front of you will help you to focus on important aspects of the business.
Who should prepare the plan? _ As a business consultant, this may sound like heresy, but I believe that any plan should be produced by the senior management of the organisation. That is not to say that the consultant does not have a role to play in its preparation. He does. Senior management should prepare the plan as they will then be able to present and discuss it, demonstrating to their audience that they fully understand their business and market. I believe that the consultant's role is to help facilitate the preparation of the plan, the consultant can help undertake the necessary research, and can cast a critical and impartial eye over the plan.
Do not try to write your business plan in one sitting. As I mentioned in the beginning, for a traditional brick and mortar business, it could take in excess of 100 hours to compile all of the information needed into a comprehensive but yet understandable document. For online businesses, probably not that long. But your final product should be well thought out, well documented and easily understandable.