Fortunately, weight loss is no mystery: reduce your calorie intake below your daily calorie burn (estimated to be between 1800 and 2300 for most adult males, not counting exercise), or increase your burn above your intake. With the help of any calorie/exercise tracker (tons of apps fot this for iOS and Android, as well as many websites), it is easy to keep track of both gain and loss of calories, and thus ensure that you stay below the required intake.
7000-7700 calories respond to 1 kilo of weight (whether from proteins, carbs, fat, etc). This means that if you create a ~7000 calorie deficit, you will have lost 1 kilo. For example, staying 500 calories below your daily calorie burn will lose you approximately 1 kilo each week, since 7 days X 500 calories = 7000 calories = 1 kilo of fat (which, incidentally, is the normal upper limit for weight loss as recommended by most doctors).
Yep, really that simple. Then why is weight loss guides one of the most common and popular forms of guides in the history of man, next to Windows support guides? Because while the theory is very simple, and the practice is just as simple in theory
, what people find hard are following the two principles that are key to weight loss: 1) Eat less and more importantly eat right. 2) Exercise.
So, the vast majority of weight loss guides that have any basis in science, deal less with the nutritional principles behind weight loss (or they explain them as quickly as I have, above), and more with motivational factors, ways to stay on target and uphold self-discipline, and ways to gain exercise and reduce calorie intake that might not be directly obvious, but are still based on the simple principles above.
It's always interesting to see all these replies touting a fast metabolism as an answer, when nutritional science is very clear on that calorie deficit versus surplus is the rule for weight loss and gain for damn near every single living human.