Remember the proverbial expression 'not seeing the wood for the trees'? You need to see the 'wood' first, then delve in and start examining the individual 'trees', meaning the individual items which you will be breaking down later. So a great point is to make sure that you have that overarching vision _ and if you cannot find one, then maybe it is an indication that you are obsessing on a few technical aspects that do not necessarily make up a whole business as you had imagined it. A business that makes sense and is going to be sustainable in the future is one that has that clear vision within which all the smaller parts contribute to make it successful.
Contextualise Your Budget _ Of course your budget will be extremely important. But sometimes people sort of pluck figures out of thin air, not giving it the context it needs in the business plan to make real concrete sense of how that budget is going to work. So it is crucial that every time you mention financials in your business plan, to really give them the correct context. When I have worked with clients in developing business plans, there has been a budget or amount set aside for example to be spent on marketing, which has been decided a bit arbitrarily. I mean with no real research, no understanding of what that amount needs to be spent on, and what that budget will truly achieve. It seems to have been put there to fill the need to attribute a certain sum to marketing. Make sure you are researching each point of your budget, make sure that you are giving it context and it makes proper sense within your overall plan.