Some people tend to be naturally underweight just like some tend to more naturally be overweight.
For those who are underweight, it's difficult to gain weight and be at what is considered a healthy weight.
For many there have been strategies that have been tried without much results.
You will need to have some effective weight gaining habits in your life to actually put on the extra pounds that you are aiming for.
This will be a combination of muscle and some fat.
The first thing you need to do is determine how many calories you are burning each day.
There are calorie calculators that will give you a rough estimate.
Also include the calories you are burning through exercise if you are doing any.
If you want to even come close to gaining a pound a week, you will need to get at least 500 extra calories per day.
Don't do this with junk food that is high in fats and sugars.
Focus on healthier foods.
Eat things like pasta, starchy vegetables, and nuts.
Many of these are higher in calories and will effect the body differently than high calorie sugary snacks will.
Focus on eating more natural foods.
You will also need a basic exercise plan.
While many avoid exercise because it's associated with losing weight, you can do exercise for the purpose of gaining muscle which will add pounds and make you a bit bigger.
This is much easier for men to do than for women.
Do weight training exercises where you are doing low repetitions with higher weight.
This is the type of routine that is meant to build muscle the fastest.
You will burn calories doing this, but in the long run you will get bigger which is your goal.
If building muscle try to add in lean meats for protein and even whey protein.
This will add extra calories to your diet and give your body something to work with during the muscle building process.