To draw a boundary on your map first select the Boundary polygon tool in your toolset on the left. To start drawing your boundary simply click once on your desired starting point and then move the mouse to your next point until you have finished drawing your boundary. The more points you add to your boundary the more detailed and smooth it will look. To finish your boundary click on the first point you created, this will close and finish your boundary.
Tip: Zooming in close and using fence lines as guides will help you draw a very accurate boundary.
See video below for a visual example.