i have written a purpose for a business plan before
Well, a business plan and an SOP do have similarities, for sure! But an SOP is more like a sales pitch, whereas a business plan is more like a list of things to do and goals to achieve.
You can google this: statement of purpose example
You will get lots of great ideas. Also, search EssayForum for "SOP"
After you read 20 or 30 of them, you will know how you want to write your own. Get inspired. In the same way you developed your own style for design when you got familiar with it, you can have your own style for writing a SOP.
However, in general I want to say I think it is important to show that you have a clearly envisioned plan that includes this particular school to which you are applying, a plan that shows what you want to be doing during the next 5 years or so. By showing that you have a plan, you prove to them that you are a worthy candidate.
Of course, you also have to describe your previous experiences. Lok at some examples of SOP and get started! :-)