Welcome aboard noora, a business plan is not so much about math as it is about providing a plan for success of your business. There is an old adage,
"Fail to plan, plan to fail."
In case it loses it's meaning due to cultural differences, this means businesses that don't have a solid business plan, review it quarterly, update it as the business climate changes, will almost always fail.
It's a little less strict for a sole proprietor business, as we don't have some requirements and overhead of traditional businesses, but it's still a good rule to live by.
To write a good business plan, talk to banks and lending institutions. Find out what they would expect from you if you were bringing your business to them to ask for financial support. It may not be that you will ever do this, but this will "bring out" what elements will need to be in your business plan.
Another is to look for examples of business plans online. Don't just copy and paste them, examine them and understand their meaning and why each part of the plan is even there. Collect ideas from several samples and build one that's unique to you.
Basically a business plan is a summary of where you are now, where you want to be in 1 year, and where you want to be in two years. Some lending institutions require a three year projection for larger businesses.
This will make you better as a business person and is an excellent lesson for your future.