Let's suppose that you are trying to describe a scene. What would the person do to get the image of the scene into her imagination if she were actually there? She see things; she would hear things, smell things, feel things. She would take these things in either in some kind of sequence or simultaneously. Her observation of the scene would create a mood in her mind, and the mood would then become the tone of the image for her and that tone would highlight certain parts of the scene and diminish others.
The writer becomes the observer in place of the reader. You become the reader's eyes, ears, nose, and body. So the first thing to do is to collect this kind of information. What do you see, hear, feel, smell? Write down specific observations. What shape are the various things you see? What colors? Where are they in relationship to each other? What do you smell? What is the scent like? What do you feel? Make the details as specific and concrete as possible. Related to being the reader's body is the notion that you supply a "point of view" for the reader. Where are you as you describe the scene? Let the reader know.
The writer also becomes the soul of the reader. What mood does the scene create? What is it reminiscent of? Why does this scene create this emotion or mood? Given this mood, which of the details recorded earlier stand out? Which seem to be insignificant? How does the mood change the character of the specific things you see and hear? As you write the description, select from the details that reinforce the mood you want to create.
The writer also becomes the consciousness of the reader. A person's consciousness is like a pilot. It directs the attention. Given the mood you want to create in the description and the selection of details you want to emphasize, you now must consider what order most strategically reinforces the mood or impression you want to create. The order for a description can be spatial, thematic, or impressionistic.
If you follow a spatial pattern, you begin usually by describing the whole scene briefly and then by focusing on one place in the scene, describing it in detail and then moving on to an adjacent place in the scene and describing it. The whole description continues as you move in a systematic description of each section of the whole.
If you follow a thematic pattern, you divide the scene into mental categories, and regardless of their actual relationship to each other in space, you describe elements that support one theme and then another theme. This pattern is really closer to classification than to description, but it is a description as long as you continue to create pictures within the subsections.
If you follow an impressionistic pattern, you imagine, given the mood of the observer, what things would draw attention to themselves first, and then second, and so on. The description would then present these details in the order that they would normally be noticed.
Overall, a good description should create a mood or tone by giving concrete details in a strategic sequence. The reader should have a strong sense of the whole scene, but should also be able to conjure up specific details easily.