Work in Layers
You're already starting to learn this. You're talking about planning your composition. After envisioning your work, composition planning is the second step. The third step, and your first layer, is when you build your composition.
Planning a Composition
First off, you need to have some idea of what you want the finished picture to look like. Not the image on the page, but the picture as an object, page and all. If you're drawing from life, this is that thing artists are doing when they hold up their fingers in little 'L' shapes to draw a box in front of their vision and close one eye. They are manually flattening their subject and enclosing it in the boundaries of their composition. They are trying to look at the finished work before it has even begun. This lets them figure out what parts can be included or excluded in the finished work. (Ex: "Maybe I like that storefront behind the lady, but including it would force me to pull back in perspective and include other ugly parts of the scene, so I can't use it... but when I zoom in, I don't get a full view of her dress and the bottom of the page cuts off at her belt...")
Another method, and this is used more predominantly in cartooning, design, and illustration, is to do preliminary sketches. Some people do miniature sketches called thumbnails, but this isn't necessary; work at whatever scale is comfortable to you. People tend to work smaller when planning because it takes less time and makes it easier to contain the whole image in the center of their vision all at once as they draw. (This is advantageous; most of the time, when working on a finished piece, you are only really able to look at some parts of the composition, while the rest is in periphery or covered by your sleeve.) In your sketches, focus on arranging the information clearly within the confines of your page. It doesn't need to look like a finished work. It doesn't even need to be recognizable or understandable to anyone but you. This isn't being made for anyone else. You are the audience for your own prep work: make it work for you. The most important thing though, is that your sketches be of a format in proportion to that of your finished work. (If your planning is done in squares and your finished work is a rectangle, your planning won't have done you much good.)
For imaginary subjects, in addition to planning the overall composition, you may need to do a lot of preliminary design work. Understanding how your subject is built in a physical way, even if it is not a real thing, goes a long way toward being able to describe it clearly within a composition. Importantly, you need to understand its proportions in perspective. Design is a whole other field of study though, you could take a whole year-long drawing course learning just about that and still have more to learn.
Constructing a Composition
Collectively, the tools we use to build our composition on the page are called construction lines. Some older American artists call them plan lines. There's a wide variety of construction lines, but they all do the same thing: tell you where to draw. And that's key, because composition construction has very little to do with what you are drawing, so much as how you will draw it. When drawing construction marks you need to be sure that:
They can be removed or covered up in the finished work without meaningful impact.
The marking and removal process will not alter the media. That means it should not damage the tooth of the paper or prevent your medium from adhering to the paper. This is why it is not advised to plan charcoal, pastel, or chalk drawings with graphite: basically nothing sticks to graphite except more graphite.
You can see the marks, even as the piece progresses to much more finished states. They should not become lost in confused marking or be forgotten by contrast. Some artists disagree with me on this. Feel free to disagree with me about this. There are strong arguments for never even removing your construction work if it is subtle enough. I feel this is far too restrictive for the young artist who, through inexperience, lacks such subtlety.
Line of Action. Though the image linked is from a (very good) cartooning guide book, the line of action is one of the most very basic of construction lines. While cartoonists build their composition around it, life artists and designers seek to find the natural line of action in their real(istic) subject. It may not always be as clear or as flashy as what we see in cartoons, but it is always there. Often, in Figure drawing, a line of action will be your first mark, immediately capturing the emotion of the pose in that first moment of striking the page. A line of action is about the "feeling" of your subject more than its actual appearance. Certainly, it follows the most abstract directionality of what you're looking at, but it also expresses the tension or movement of that subject. A fat, saggy tree, even with a perfectly straight trunk, may evoke a mostly straight, but pot-bellied line of action, possibly with a greater line breadth near its base to emphasize weight. A line of action has an anthropomorphic effect on your work, allowing you to breathe your life as an artist into your work, by imbuing it with your emotional investment in the subject. Certainly not every little thing should be reduced to an abstract line on the page, just the main subject matter, those one, two, or three objects which the image is "about". Lines of action are best at plotting the position and pose of living things. Inanimate things often have bland and arbitrary lines which give us little to no useful information.
Guidelines. Guidelines are an abstraction of design within a composition. Basically, you are simplifying a subject down to its most fundamental structures and drawing only those, then elaborating upon that initial state in layers of increasing detail. An airplane might start as nothing more than a triangle drawn in perspective. A tree might be nothing more than a venn diagram with a stem. A person might be less than a stick-man. A lot of how-to-draw books start here, as if this is all you need to know to build a good composition. The honest truth is that if you make a compositional mistake at this step and don't correct it, you will waste your time finishing a mistake. When drawing guidelines, stop and look at the total work before you move on to the next step. Know what your guidelines represent and envision what the finished work will look like if you follow them as they are now, not what you want them to be. Be brutally honest with yourself here: guidelines are easily moved, and unworked paper is cheap. It'll hurt less to correct things for the better now, than to accept a total loss later. Now, as for what your guidelines should look like? That's a much harder question to answer. There's generally three styles, but there's endless variations unique to each artist. First, you have the ever-popular how-to-draw guidelines, called modelling or "sausage drawing". This generalizes things down into simple geometric blocks or blobs. Some artists draw 3d blocks. Others just draw flat shapes. This method focuses on the contour, or silhouette of the subject, without saying anything about details. This is why this method is so popular among cartoonists: When making an iconic image, silhouette is everything. If you use vaguely 3d block shapes, you can tell yourself information about the subject's relation to perspective, or even draw parts of the subject in perspective while planning. Then you have skeleton or wire frame drawing. This method summarizes the subject down to just a few lines showing the arrangement of its parts in proportion to each other and (somewhat) in perspective, but offers little to little information about contour or volume. Wire frame techniques often show the proportional relationships between fine details. You will most often see it on heads, hands, and feet, as they are very complicated things. Finally, you have points. This reduces a subject to nothing more than a collection of dots on the page. This is very uncommon, and also very hard to show clearly. I cannot find any examples to link you to. Basically, each dot represents something different and important, known only to the artist. A dot could represent a start or end of a line, a boundary for a contour, a vanishing point, the extreme end of an object, the center of a thing... the list goes on. While guiding dots tell the audience nothing, in the mind of an artist who tends toward this method, they mean everything.
The wide variety of guidelines exists because guidelines are all personal mental work. They are not the finished product. They are your tools to make that product. Every mind, every hand, every artist is different, so every artist will need to develop unique tools to help them achieve their goals. While watching and imitating other artists tevhniques can lead you in the right direction, the correct path is the one you cut yourself. Do what you understand best, what feels best, and what works best. Nobody will ever see any of this to judge you by it anyways, so all that matters is its utility to you and your work.
Perspective. This is a whole field of drawing. Drawing in perspective is not easy. It is a complicated, analytical skill. That said, if you learn it, you should use it, (or at least abuse it) it will benefit your work. Generally though, perspective is a geometry trick used to create the impression of depth by drawing all marks relative to 1-3 "vanishing points" representing the absolute infinite distance ahead. This can be simple or bloody complicated, depending on your subject.
Chunking. OK, so I made that up. There isn't a word for this, because nobody talks about it, but I've seen countless other artists do this, and I do it myself. Chuncking is the technique of taking a big problem and breaking it down into smaller ones. In college art courses, you're likely to run into this philosophy that all artworks are like a problem to be solved. The problem is the "how" of turning an idea into a thing. The easiest way to "solve" the problem of a drawing is to take apart that blank sheet of paper, dismantle it into smaller, more easily managed pieces, and work on them incrementally and sequentially. Really, all forms of construction lines are part of this chunking process. We draw a circle as a placeholder for the head; a cross as a placeholder for a face; some curvy tetrahedrons as placeholders for the shape of her hair, etc. Chunking, when done in isolation of any other construction technique, deals with the page itself. You draw as if with a knife, slicing up the page into sections. The tall skinny kite is the person I'm drawing. This box is a house. This line is the horizon. This circle is a tree. Etc. We summarize the scene and assign some of our page to each element it contains, divvying up the page as if a resource.
The Next Step
So, once you have completed your construction and you think the composition is sound enough to make a finished work, it's time to begin the finishing process. Add general details first, and apply that small level of change to every chunk of the page. Start from the beginning again and add another level, this time get a little more detailed. Do this over and over again. As guidelines become irrelevant because the drawing can stand on its own, replace them. Just keep going until it's done.
