Create four lines in your rectangular space. Two horizontal lines will divide your rectangle into three equally sized horizontal rectangles. Two vertical lines will divide your rectangle into three equally sized vertical rectangles. Creating a line may mean sketching it in light pencil on a piece of paper if you are drawing. If you are using a camera, then you will have to mentally visualize where these lines are (some cameras feature a digital rule of thirds option that creates the lines for you on your display).
Mark or take mental note of where the four intersections of your four lines take place. These intersections are sometimes referred to as power points. Elements on or close to power points tend to draw more focus from the viewer.
Arrange the elements of your piece along the lines you created and on the power points. This will create a visually appealing composition. A common mistake is to place an intended subject of focus directly in the center of the rectangle. However, you will see from the rule of thirds grid you created that a subject gains more power by being placed along one of the lines or near an intersection of two lines. Experienced photographers know, for example, that a horizon should never cut across the center of a photograph. Placing it along the upper or lower third line is more interesting.