1. Eat six small meals during the day rather than two large meals. Also drink your water before meals. It can fill your stomach and make it harder to get in enough calories.
2. Foods
Milk, protein smoothies, rice, salmon, nuts, potatoes, red meat, avocados, whole egg, dark chocolate, cheese, whole fat yogurt.
3. Workout
Workout at least three times a week by lifting weights. Heavy compound exercises like squats and deadlifts are great exercises to gain muscle weight. They stress your body and muscles with heavy weights. This increases your testosterone levels and makes you build muscle.
These are the basic tips on how to gain weight healthy. Hope it helped let me know what you guys think.