Click the 'Draw Line' button which is located on the DRAW ribbon.
TIP: You can also initiate this function by using the default Hot Key ' L '
Now click on the screen at the point you want to start the first point of your line.
Next click where you want the end point of the line to be. The line can be extended in another direction if needed.
To finish drawing your line, press your right hand mouse button or press the ESC (escape) key on your keyboard.