Losing one pound of fat is not dificult. If we can burn 3500 more calories than we eat, one pound of fat (which is stored in your body) will be converted into energy. So total fat stored in our body will be reduced by one pound. Now how can we burn 3500 more calories? We can eat less food, hence we can reduce calorie intake or we can burn more calorie through exercise, walking, running or other physical exercises.
More realistic way is to combine both options, ie, by taking less food and by doing more physical activities. Losing one pound in a week or in a day? Now the question is how much time you need to lose one pound of fat! One day or one week or months? It really depends on you. You can lose one pound of fat in a week, or in two weeks or even in a day. If you eat just 1000 calories and burn 4500 calories on a day, then on that day you will lose one pound of fat. But taking only 1000 calories is dificult. Burning 4500 calories in one day is even more dificult.
You need to run 25/30 miles or walk around 50/60 miles on that day. Unless you have an athletic body (which I suppose you don't have, otherwise you wouldn't visit website on "how to flatten stomach" or "How to lose Fat") it is highly improbable that you will run or walk that many miles. So it is not realisitic to set a goal to lose one pound of fat a day. Many of your friends or colleague may have told you their stories of losing 2/3 pounds of weight in couple of days. So are they telling lies? No they are not. They may have lost few pounds off their weight and they are happy to shear the success stories with their friends. But what happend then, did any of them lose 30 pounds in a month, and finally become thin and lean! I don't think so. I hope you noticed I mentioned "losing 2/3 pounds of weight", losing 2/3 pounds of "Fat".
There are many fad diets and pills, which will drain out initally lot of water from the body. So they should be happy to look at the weight scale and to shear their success with others. If you start fasting and take very few carbohydrate, you will also lose mostly water on first couple of days. So you will find you lose couple of pounds in one or two days. But after few days the lost weight will come back. So we need to realize the difference between losing weight and losing fat. Losing fat is what we are after. As I was discussing, losing a pound a day is not realistic. Losing a pound a week is within reach of most of the people. Even you can go for one pound in two weeks. See if you lose one pound in each two week, in six month ( 26 weeks), you will lose 13 pounds, ina year 26 pounds, most of the people don't need to lose more than that. HOW?
Now we reach to the most important point. How we can lose one pound of fat in a week. Answer is simple, you have to eat less or be more active physically, or do the both. Suppose you are burning usually 2000 calories, and eating around little more or same. So you have to eat 500 calories less each day.
So in a week you have deficit of 500X7 or 3500 calories. Which means you will lose each days 1/5th of a pound, and at the end of the week, cumulative weight loss would be 1 pound. If you think eating less is dificult for you, you may rather increase the physical activites, keeping the same food intake you had before. To spend extra 500 calories a day, you may walk each day 2 hours (at three miles per hour speed), or 6 miles. Or you may just run, 50 minutes a day ( at 5 mph). Or you make a combination of the both. Eat 250 calories less, and walk 1 hour or run 25 minutes. So you see losing a pound a week, is possible, but you have to be firm on food intake or physical activities. Losing a pound in each two week may be much easier and more realistic goal.