The easiest way is to drag a subordinate shape from the stencil and drop it
on top of the manager shape. Visio will automatically connect the two shapes
and place the new shape in a reasonable place.
Alternatively, you can drop a subordinate shape anywhere on the page and
connect the two shapes using the connector tool or the dynamic connector
from the stencil. Visio will infer the reporting relationship based on the
relative positions of the shapes on the page.
There is lots of information in the Visio help file. Just click on the
"Help" menu item and search for "Organization Chart".