Eat! Nothing else will get you there except eating over the amount of calories your body consumes every day. Lets say you need 1800 kcal a day. So in order to gain weight you need to consume lets say 2200 kcal, but keep in mind not to go over the board with carbs and fats. Try to consume only omega 3-6-9- fats and not saturated fats. And no sugar, or atleast as low as possible.