Gaining 12 lbs of muscles as an ectomorph is incredibly tough. The short answer is I researched nutrition to microscopic details, crafted the right recipes and basically only ate that for the month, leading to 4,500 (perfect) calories a day. I was doing full-body workouts 3 times a week + 100 pushups, 100 squats, and 100 pullups every day. It was the most intense month of my life.
Though I did manage to replicate these results twice. The first time I actually gained 15 lbs.