Consider all of the items that impact your business
Don't try and knock your business plan up in just a couple of days. You really need to consider all of the items that could impact your operation. If possible, talk to other people in the industry or potentially in related industries and find out what their major costs are and how they manage their operation. Once you have done a bit of digging you can create a proforma P&L of all of the items that you think are relevant.
A good business plan should include a really good narrative in terms of what you intend to do and how you intend to do it. You should really go into detail about what you perceive as your opportunities and what you perceive as your threats. Your narrative should work with the numbers to really tell a story.
Get a second opinion
Don't try to do it all yourself as you are more likely to miss items. Talk to as many knowledgeable people as you can. If you do not have a financial background then it could be a good idea to talk to an accountant or your bank about how well your financials stand up.
Revise your business plan regularly
Creating a good business plan is not just about formulating a plan at a specific point in time and then locking it away in the cupboard. A good business plan should form the starting point for your budget and you should then review your assumptions every 3 months or so. You should then revise your plan as you get more details of what reality actually looks like.
Share it with your bank
Once you have created your bullet proof business plan you should think about sharing it with your bank or your finance provider. Your bank will be a lot more confident with your ability to deliver if the see that you have actually put in the time and effort to prepare a proper business plan.