Read Corporate Modeler's help pages. The illustration (right) shows the best topic to start at:

Review the slides from module 1 of the Corporate Modeler Foundation course; these mainly cover how to manipulate the diagrammer (e.g. how to draw a diagram, align shapes on the diagram, move shapes, and so on).

The content of your diagram, however, is where the real work begins; this is where you need to interpret your source data and transfer it into a meaningful diagram that also accords with The Environment Agency's modeling standards.