Here is a simple method for creating depth in drawings (and paintings for that matter) by using tone. If you are lucky enough to be able to stand in the countryside and look towards the horizon you will notice a number of things useful for drawing and painting. Perhaps the most important is the fact that as everything recedes towards the horizon they become smaller, lighter in tone and less detailed. Objects have softer edges and the difference between the lightest lights and the darkest darks becomes much narrower as you get closer to the horizon. See my video on Landscape drawing – Creating Depth for more information.
With these observations in mind, start a landscape drawing. When you reach the point that you wish to add tone try this idea. Look in the foreground and select what you consider to be the very darkest tone. Scribble this tone in a very small part of your foreground and now look to the middle ground, again look for the darkest tone and scribble this tone as described above. Do the same with the background. You should end up with three small areas of tone, the darkest being in the foreground and the lightest one in the background.
Now you have a key to begin adding all your other tones, because you now know that no tone in the middle ground can be darker that the middle ground tone you have drawn already so every tone has to be lighter. Do the same with the background and foreground. You should end up with a whole series fo tones gradually getting lighter as they recede into the background. You can use this idea for painting also.