For some one who is under weight, how he/she can put on weight in a healthy manner?

eat more, alot more. I eat 6 meal a day, keep away from the fat.

Well it depends on the reason why someone is underweight. He or she should take healthy nutritonal food to get rid off being underweight.

In order to gain weight, it is important to eat foods that are rich in carbs. Rice is a good sample of it. Eat as many as you can, eat many times, and always eat.

Well , it depends on your stats and methabolism.
Check this article to determine what is your body specific and what you need.
Body-specific Training