Calories…. you have to determine how many calories that you need to sustain current weight and go from there. Add more calories than you need to sustain weight, example an extra 500 to 750 calories per day.
That ectomorph stuff really hasn’t been backed up by research. Anyone can put on weight and muscle, regardless of body frame size.
The easiest way to add calories would be to eat a hearty cereal such as Granola, and add juice to your diet to increase calories. Choose starchy vegetables – which have more calories. Add nuts as a snack an a couple of high calorie smooth/protein shakes per day.
To be really detailed, a Dietitian would need to get a lot of detailed information about you and your workout routine.
Hope this helps