When painting out of doors, it is important to find a subject that inspires you, and then orient yourself to the sun in relation to nature. Find the cardinal points. Remember the sun moves on a southerly path to the west. Note the time of day, and anticipate the effect in an hour or more. Ask yourself what the light will do to your subject matter later in the day. This is one of the most important goals for a plein air painter; to portray the time of day the scene was painted. The goal is to paint a 20-minute period of time, or an effect of light upon the desired scene.
There are two main effects of light, back light (looking east, south or west into the sun) where the shadow effect on your subject makes a strong statement, or front
light, the effect of light in which the sun illuminates the subject evenly. Once you have considered these points, you are ready to compose your picture. Decide whether your subject is vertical or horizontal and hold the canvas accordingly. Next, consider nature's scene using a viewfinder, or at least realize where your left and right edges are, as well as top and bottom.
Composition is personal and there is no limit as to what one can do, as long as it looks right to the trained eye. Balancing the elements is the essence of good picture making. Elements such as lines and shapes, as well as masses of light and dark, must all be balanced; that is, one mass must relate to the proportion of another. Color relationships should also be considered part of this balancing act. It is entirely a matter of visual balance: ask yourself, does it feel right? The balanced elements must extend further to ensure good composition as well as movement, or rhythm. This is the goal of all art forms, whether it be painting, poetry, music, theatre, sculpture, or architecture, etc. A suggestion would be to 'spot' a simple composition by finding the center of your canvas and placing the focal point or interest off center. The positive and negative spaces should be unequal. Thus finding the center and spotting may help you to understand this concept of positive and negative space. Pay attention to the negative space, or that which makes the positive space come to life. An example might be to see the sky as more important than the silhouette of the earth. When drawing the shapes of your scene in nature with brush and paint, try seeing the masses of tone connecting one another rather than the objects that are the tones.