1. Regularly (three days per week seems to be about right) lift heavy weights using whole-body movements (three sets of five reps of back squat, bench/shoulder press, and deadlift at near maximal weights for you is the ball park of what you're shooting for).

2. Eat lots of calories with adequate protein (30% seems to be good).

If you're a really hard gainer and that still doesn't work for you, try adding a gallon of whole milk a day. That will definitely do it--whole milk evolved for the purpose of putting a lot of muscle on baby cows quickly. I wouldn't say it's a particularly healthy thing to do long-term but that will almost certainly make you gain significant amounts of muscle.