Don’t smoke! Smoking can make you lose weight. Smokers tend to weigh less, and quitting smoking eventually leads to weight gain.
So you have better be away from any activity that may ruin your diet and prevent you from gaining weight.
You may face some difficulty to make your weight higher. Most of the time, you may need to force yourself to eat despite feeling full.
The process will take a long time, and you need to be persevering and have faith in yourself if you want to succeed in this fitness journey.