I recommends the following basic principles to keep in mind if you wish to gain weight in a healthy way:
increase your appetite by eating more to gain weight. But do it step-by-step and not by stuffing yourself too much at one go. Add one extra chapatti or more rice than you normally consume, or you even start with fruits by eating an extra one every day.
Carbs are needed to gain weight but include healthier options like banana, potato, mango, whole grains, rice, etc. Instead of sugar-rich foods, chocolates and other junk foods.
In order to increase your muscle mass, it is essential to do weight training 4-5 times a week for an hour. You can vary the intensity depending on your capacity.
Consume protein-rich foods after you workout to increase muscle
mass and eat good fats like nuts and seeds in your diet
. Eating a handful of pumpkin or flaxseeds is a good way to give your body the fat intake it needs. Though junk foods like vada pav or burgers are loaded with calories, they only increase body fat and not muscle mass which is not the right way to go about.
Here is a healthy weight gain diet plan which includes the right foods one should consume to put on kilos in a healthy manner. But along with this, you also need to engage in physical activity for at least 30 minutes every day. It is also essential to have at least 6-8 glasses of water to flush out toxins from the body.