... or "Studying for Dummies". Here are some simple notes on how to study effectively. This is not a complete cookbook but hopefully there are some useful hints. Note that most of it was written while thinking of studying math/engineering so some specifics might not apply to you. There is a good reason why I chose math: It requires all three different processes:
Many fields of study just focus on one or two of these, e.g. languages are mostly reading and training (reading texts and books, training vocabulary). There is not a lot to understand; grammar always has some fuzzy rules that require learning them and all exceptions to the rules by heart. In sports the focus is on training. There is one small set of rules for a sport which rarely changes and which can be learned quickly (look at how many people who know all the rules of soccer or baseball). Compared to that, there are relatively few people who know "all" the rules of mathematics or some field of engineering.
I have met many people who studied a lot (or at least said they did) and still often failed their exams. Let's just say they really spent a lot of time studying, or thinking that they were studying. What were they doing wrong?
I have also met a lot of people who seemed to be able to do thousands of things, they were active in a sports club, had a girlfriend/boyfriend, traveled the world, volunteered whenever there was an opportunity and still had the best grades, apparently not studying at all.
If you are studying a lot - let's say 4-6 hours a day (which does not count lectures or classes) - and you are still having trouble keeping up you are probably doing it wrong.
This is a frequent mistake. People "study" till 3am, or they stay up watching movies or go out and get wasted, getting back some time in the morning, or they spend a lot of what should be sleep time with their partner. All of these have a problem: it is impossible to learn on an average of 4 or 5 hours of sleep daily. Going to parties and getting wasted, having a hangover for the next day or two is all fine - but not if you know you have an exam in two weeks that you really need to study for. You will waste three days while your body tries to go back to a state where studying is possible.
The key really is sleeping regularly. Taking a 3-hour-nap at 2pm, then sleeping 4 hours at night is not the same as 7 hours of sleep at night, every day, during the same time. You will find yourself dizzy, having trouble to say what time it is and you will still feel exhausted and/or unable to sleep at night. Irregular sleep schedules are for the holidays and are incompatible with studying.
Now I often hear, "but I do not have time, i need to study late in the evening/at night." You should ask yourself what you are trying to do by "studying" at these hours. Are you actually trying to study or are you panicking, trying to make up for time that was spent otherwise? Or are you maybe trying to study to get an excuse for after the exam, "well but I studied! I did so much! Why did it not work?" (I hear that a lot.)
Next time when you are about to "study" late at evening/night or when you are tired, try to realize that you are probably not in a state to process information as fast as you could after you have had a good night's rest. The actual results will vary from person to person and also depending on the subject of study, but (personal experience) you might be three times slower or you might not even remember anything for some particularly difficult subjects (like maths). This is what most people do who "study a lot" but still fail a lot. These people, when asked what they studied last evening, mostly cannot even say what they studied, much less remember anything in particular. That is a sign of wasted hours spent "studying".
Remember, you could just sleep for three hours more, get up in the morning and do everything or more in a single hour. Those energetic people who always pass exams with flying colors, even going to parties, they probably have a very good intuition or strict timekeeping for getting enough rest and studying at the right time.
Tl;dr: If you are tired, your brain is a lot slower. Sleep regularly and you can do a lot more in a lot less time.
Have you ever been at a friend's appartment and wondered where they did their homework? Every table full of "stuff": laundry, books, used dishes, etc. Of course this strongly depends on the field of study, but studying often involves reading and writing: you need space to read and write and you should not need to clean up before studying. If you need to clean up, chances are that you end up cleaning the whole appartment for hours, spending time that you are fully awake and mentally capable of studying doing simple things like putting away dishes, doing laundry and cleaning tabletops, all of which can be done when mentally exhausted.
It is not easy to say how much space you need, personally I have found that I need a book, paper for writing and space for extra stuff (like a calculator). Whatever you need, you need to be able to put these things side by side effortlessly. If you need to put your calculator onto the same book that you need to solve a problem then there is not enough space.
An advice often heard concerning studying is "get comfortable". Most people then make the mistake of associating "comfortable" with "cinema" or "movie night", get crisps and a drink and lie down somewhere - this is not what is meant by comfortable here. What is meant is that you should be able to read and write comfortably. You cannot do that while lying down or sitting on the couch in front of the couch table (except if it is really just reading, but if you are not taking notes then are you really studying?).
A controversial topic is music. Personally I prefer studying without music. Music can be inspiring and motivating but also gets you sidetracked easily. You might find at some point (especially when reading) that you are actually just listening to the music and cannot remember what the last paragraph was about. Try solving some math problems while music is playing, you will probably find that it makes it hard to concentrate.
Finally, other people might become a problem. Particularly if they are having a good time, watching videos or a movie, maybe having a beer or two, laughing loudly, interrupting you to show you "the best part", the list goes on. This kills concentration. Either study alone or in a quiet room - the library is often a good place for that.
Tl;dr: You need enough space to put down your stuff without rearranging everything all the time. Do not lie down and/or "study" on the couch - it does not work. Avoid listening to music while studying. Do not "study" at places where other people are having a fun time.
Eating: You will need your brain while studying, and the brain uses a lot of energy. Trying to study while hungry is at least as ineffective as trying to study when tired. You should not feel stuffed either, since that makes you tired. Whatever you eat, it should not make you tired and it should last a while. Also, being on a diet (and therefore constantly hungry) while studying will not work. You can diet after the exam.
Avoid eating while studying. It is distracting and you will get fat-, juice- and other stains on books (that might not even be yours); besides, your stomach will permanently be digesting, which causes the blood flow to concentrate in your bowels, leaving less for your brain. Also, constantly eating makes you fat. If you are permanently hungry, chances are that you are not eating appropriately. Remember that whatever you eat should not make you tired and it should last a while (at least three hours).
When to do what: Reading, understanding or training? This is actually simple to figure out, at least for math. You cannot train (i.e. solve exercise problems) if you do not understand the subject. A sign for this is looking at the solutions all the time and/or copying from the solutions while trying to solve exercise problems. Therefore, the first thing is reading and trying to understand the subject. If you have problems understanding certain things, get someone to help you.
Secondly, people often read when they should train. You cannot train for an exam by looking at pre-calculated exercises. Even if you can perfectly follow the train of thought that is in a solution, there is no guarantee that you can replicate the results. (Watch someone dance: it is easy to follow, you can see all the moves, everything is logical. You will still not be able to dance just after seeing someone dance.) Nobody loves doing exercise after exercise, but it is exercise that makes perfect. If you tell someone you studied for an exam, think twice: did you really do all three, learn, understand and train?
All of this was probably very obvious. There are more advanced things like estimating how long you will need to study for a certain exam, making a timetable and keeping to it, learning techniques etc. for which there are plenty of ressources. However, most people who fail an exam have problems with one or more of the above "simple" things.
After failing an exam, people try to convince their surroundings that they studied when in fact they just skipped through a book or their notes a few times, were often distracted, too tired or hungry, or they blame the teacher, the assistants, the exam problems. These people are just trying to justify their failure for themselves. Everyone who passed the exam and probably really studied knows, and it just makes them look stupid.
So you tried, but you are just too lazy, you cannot be consequent enough, are constantly sidetracked, cannot keep your desk clean or get your roommates to be quiet?
Get help.
Really, do you want to pass that exam, finish your studies? Pay someone to help you. There are plenty of people and institutions who help people study correctly. Remember, there were others before you that managed to do it. Your life is what you make of its opportunities.