Don't get too many things into one landscape. The spirit of the time and place is what will make the beauty of it, not the details nor the mere facts. This spirit you will find in a few things, not in many. Having found which lines and forms, which masses and relations of color and value, express this, the more carefully you avoid putting in other things the more entirely you emphasize the quality which is the real reason of existence of your picture.
In studying landscape, work for one thing at a time. What has been said of sketching and studies applies here. Landscape is the most bewildering of subjects in its multiplicity of facts and objects and colors and contrasts. If you cannot find a way to simplify it you will neither know where to begin nor where to leave off. I cannot tell you just what to do or not to do, because no two landscapes are alike. Recipes will do nothing in helping you to paint. But there is the general principle which you may follow, and I try to keep it before you even at the risk of over-repetition. In no kind of picture can you drag in unimportant things simply because they exist in nature. In landscape more than elsewhere, because you cannot arrange it, but must select in the actual presence of everything, you must learn to concentrate on the things which mean most, and to refuse to recognize those which will not lend themselves to the central idea.
=Selection.=--When you select your subject, or "_motif_," as the French call it, select it for something definite. There is always something which makes you think this particular view will make a good picture. State to yourself what it is that you see in it, not in detail, but in the general. Is it the general color effect of the whole, or a contrast? Is it a sense of largeness and space, or a beautiful combination of line in the track of a road, or row of trees, or a river? Perhaps it is the mass and majesty of a mountain or a group of trees. Something definite or definable catches you--else you had better not do it at all; and what that something is you must know quite precisely, or you will not have a well-understood picture.
When you have distinctly in your mind what you want to paint it for, then see that the composition is so placed on your canvas that that characteristic is the main thing in evidence. With this done it is a very easy thing to concentrate on that characteristic, and to leave out whatever tends to break it up or distract from it. This is the only way you can simplify your subject. First by a distinct conception of _what_ you paint it for, then by so much analysis of the whole field of vision as will show you what does and what does not help in the expression of it.