First, be sure to eat more. No matter what you eat, you should double your food intake and eat six meals a day.

Second, quit smoking and drinking, and ensure 10 hours of sleep. Try to take a lunch break if possible.

Third, proper exercise, such as weight-bearing squats, frog jumps, sprints and pull-ups, will help you to increase your muscles. Give priority to with heavy load and few times of strength exercise. Long-term endurance exercise consumes weight, which is not conducive to gaining weight.