To draw a line on your map first select the line you would like to draw from your toolset on the left. To start a line simply click once on your desired starting point and then move the mouse to your next point until you have finished drawing your line. To finish your line click on the last point you create.
Tip: The more points you add to your line the more detailed and smooth it will look. The closer you zoom in to draw your road or fence the more accurate it will be for your share map recipients.
See video below for a visual example.