You need to focus on two elements. First is your food intake and second is your workout plan. For weight gain, one must eat more calories than one burns. But the proper approach isn’t to just starting eating double the food you currently eat.
The idea is to start changing things gradually. For example, if you are eating a sandwich with 3 slices of turkey, add a 4th slice the next time. Don’t try to eat two sandwiches. If you can count calories, you want to increase your intake by 100 to 200 calories per day. You can raise this gradually from week to week. By week 6, you can be at 300 to 400 calories higher per day.
If you want to build muscle, focus on more protein rich diet. Aim for 1 to 1.5 grams of protein per pound of body weight.
Video by PictureFit