Lay out what people are supposed to be doing before they start work. If you do not have a clear goal in mind of what you want people to do, it is likely you will not get the results you want. By putting them down on paper you have something to discuss and clarify.
Realize that jobs often change over time and that the description may need to change with it. If you have people currently working that do not have job descriptions, have them write one for their job and you write one too. Then compare and contrast your versions, it may surprise you.
What is your job description? What are the descriptions of the people you work with?