The Wall button allows you to draw straight and diagonal lines on your map. Click where you want the wall to start and drag the mouse to where it will end. As you draws lines they will be added to a list underneath the Wall button with the most recent line being at the bottom of the list. To delete a wall section select it from this list and click the Delete Selected button at the bottom of the pane.
Doors are drawn the same way as walls using the Door button. You can drag the mouse across multiple tiles to make doors larger than one square. Doors will also be added to the list in the middle of this pane and can be selected and deleted just like walls.
The Eraser button will allow you to remove a small part of a door or wall that you've drawn instead of deleting it entirely using the Delete Selected button.