Choose a simple business process you are familiar with as a worker or customer.  Use the Flowchart-BPMN template in Visio to create a Level0 diagram of the process.  Include a sub-process in your diagram.

For help see:

http://office.microsoft.com/en-us/visio-help/bpmn-diagramming-basics-RZ102712773.aspx?CTT=1&client=1