How to gain weight
The Process of Gaining Weight
To gain a pound, you must eat 3,500 calories in excess of what you burn. By adding 250 to 500 calories per day you can increase your weight by 1/2 to 1 pound in a week. Large servings of unhealthy foods, such as sugar, ice cream, chips and snack mixes, do little to contribute to healthy weight gain. Gorging on these foods, even for a week, encourages you to develop poor eating habits that can lead to metabolic disturbances and chronic disease no matter your body size. Use your home kitchen to craft healthy meals and snacks, rather than relying on processed foods grabbed from your pantry.
Add Extra Calories to Meals
Boost your calorie intake at every homemade meal. Increase portion sizes, but if you have a light appetite, add calories to the food you do eat. Sprinkle nuts over your morning cereal; toss avocado in salads and spread into sandwiches; stir pasta with olive oil before adding sauce; and choose starchy, rather than leafy, vegetables, as side dishes. An ounce of nuts adds 167 calories, a tablespoon of olive oil, 124 calories, and a cup of avocado cubes, 240 calories.
Increase Your Protein Intake
Use the week to get into the habit of consuming more calories from protein. To gain healthy muscle mass, explains Dr. Melina Jampolis on CNN, consume a total of 0.55 grams of protein per pound of body weight every day. So, if you weigh 120 pounds, consume at least 66 grams of protein daily. 

Eat a cup of roasted chicken meat for 234 calories and 35 grams of protein; 3 ounces of flank steak for 224 calories and 23 grams of protein; or 1 cup of black beans for 218 calories and 14.5 grams of protein. Nuts, nut butter, cheese and yogurt are other ways to boost calories and protein intake.
Eat Snacks Often
Use your time at home to graze regularly. Several small meals and snacks may seem more manageable than three large meals. Dried fruits, seeds, cheese and nut butter are nutritionally rich and calorie dense. Snack on a 1/4 cup of raisins for about 110 calories; spread 2 tablespoons of peanut butter on a medium banana and consume almost 300 calories; and have an ounce of cheddar cheese with just seven woven wheat crackers for 233 extra calories. Add one or all of these snacks daily to pile on a pound in a week.
Drink Up to Gain Weight
If eating more fills you up too much, try drinking your calories. Milk, 100-percent fruit juice and homemade smoothies add calories between meals. Whiz up a high-calorie smoothie containing a whole banana, frozen yogurt, milk, juice and nut butter or flax seed in your own kitchen. Include dried milk powder or protein powder to boost the calorie count of plain milk. 

Avoid drinking copious amounts of beverages with meals, though.The liquid creates temporary feelings of fullness in your stomach so you may eat less solid, calorie-containing food. Save drinks for before, after and between meals.
Light Exercise to Gain Weight
While it may seem counterintuitive, moving more actually assists you in your week-long quest to gain weight. You can’t add considerable muscle to your frame in a week, but you can start a program at home that will help you gain more lean mass over time. Use the week to practice body-weight exercises such as squats, pushups and lunges in your own living room. 

Inactivity causes your appetite to suffer. Just a little exercise at home can stimulate your appetite so you want to eat more. A simple walk around your neighborhood or pedaling a stationary bike in your home gym for short bouts of 20 to 30 minutes suffices.