Gaining weight is simple, it's the other way around. You need to eat more calories than you burn. So just like with losing weight, track everything with a scale and a food log (like myfitnesspal). See how many calories you need to start gaining weight and start tracking. Maybe you'll notice you're not eating at a surplus at your age, activity level, etc. And just like losing weight, it will take a while, depending on how much your surplus is. You might gain 1-2 lbs a week.