Select some picture; put it on a table and then look at it for two minutes. Concentrate your attention on this picture, observe every detail; then shut your eyes and see how much you can recall about it. Think of what the picture represents; whether it is a good subject; whether it looks natural. Think of objects in foreground, middle ground, background; of details of color and form. Now open your eyes and hold yourself rigidly to the correction of each and every mistake. Close eyes again and notice how much more accurate your picture is. Practice until your mental image corresponds in every particular to the original.