The first rule of trying to put on 20 lbs. of fat is not telling your overweight friends about your plans. Very few people go on a diet to gain weight, and you may encounter some resentment on that front. In a nutshell, gaining weight is simply the reverse of losing weight. You have to eat enough calories that you exceed the calories you burn each day. To prevent the new mass from becoming lean muscle, you also need to limit your exercise.
Check with your doctor about your fat gaining plan. Gaining that much fat is rarely healthy. In cases where it is necessary, this is usually the result of a medical condition that requires supervision.
Journal what you eat for two weeks. Keep track of what you eat, how much you eat and approximately how many calories each meal contains.
Set a goal for how soon you want to gain 20 lbs. Health counselor Maya Paul advises against diet plans that change your weight by more than 2 lbs. per week.
Divide 70,000 by the number of days you have before your goal for gaining the weight. This is the number of calories you will need to add to your daily caloric intake. For example, if you gave yourself 10 weeks -- 70 days -- you would need to add 1,000 calories per day.
Consider using protein shakes and weight gain supplements to pack in extra calories. Although these are commonly used by bodybuilders, they won't build muscle unless you exercise.
Maintain the same level of activity as you always have, even with the extra calories. If you burn more energy, you'll need to adjust your caloric intake.
Weigh yourself once per week and adjust your diet to suit your results. If you're gaining to slowly, add more food. If too quickly, reduce your intake.