You can find the standard Data Flow Model Diagram template in the Software and Databases category.
If you create a new document from this template, you will see that there are just four masters on the Gane-Sarson stencil, and there are no rules associated with it at all.
If you then drag-and-drop just one of each shape onto the page, you will see that the graphics are not complicated either.
You now need to review the current shapes, and one way to do this is to create a quick report in Visio. I started by reviewing ...