Marianne and Allie have some useful thoughts. I’ll just add that the philosophy behind a well-formulated ketogenic diet is to work with the body’s hormonal response, in order to reverse metabolic disease and obesity. The key is keeping carbohydrate low enough to avoid stimulating an adverse (high) insulin response, since insulin is the principal hormone involved in causing the body to store fat (among its many other duties). Excessively high insulin levels also damage the body in various ways.
So the key to eating ketogenically is not to count calories, but to count carbohydrate. Keeping below your daily threshold is important to keeping insulin low, because carbohydrate causes the largest insulin response. Protein has a lesser effect, and the degree of the effect depends on the amount of carbohydrate in the diet. On a low-carbohydrate diet, the insulin effect of protein is offset by increased levels of another hormone, called glucagon, which helps stimulate the production of ketones in the liver (too much insulin, on the other hand, shuts off this process). Fat has almost no effect on insulin whatsoever, so it is a safe source of calories to replace the energy from the carbohydrate you are no longer eating.
So keeping insulin low allows fat to leave our adipose tissue, but the body will still hang on to its fat store if we don’t eat enough. I call this “famine mode,” because the body will hang on to its reserves for as long as possible, to keep us going through the period of short rations (interestingly, fasting—eating no food at all—has a much different effect on the body). On a low-carbohydrate diet, insulin is no longer interfering with the hormones that regulate our appetite, so that our appetite then becomes a reliable guide to how much to eat. Eat to satisfy your hunger, and don’t eat again until you are hungry again, and you might be surprised at how the fat melts off.