Well, working out would be the number one criteria. If you don\'t cause hypertrophy, you don\'t build. But if you have a good workout routine, then I\'d reccomend eating alot, from healthy sources (monounsat fat, complete proteins, low glycemic carbs). Also, after workouts be sure to take in around 500 kcals from high glycemic carbs (sugars, best one is malto dextrin). After workouts your body is low on glycogen (your bodies fules source) so it will start breaking down muscle mass for energy, this is called catobolism. We don\'t like this, we want our bodies to produce muscle not eat it, so you take in a shit load of fast absorbing carbs to stop this and make you body go anabolic (building muscle).
This is the basis of muscle building, cause hypertrophy through accute stimulace, and provide your body with the nutrition it needs in order to hyper compensate.