These are the absolute, undeniable basics, which no informed PT or fitness blog could argue.
Lift Heavy Weights. Heavy weights stimulate damage in your muscle fibers, thus forcing your body into responding with muscle growth. Essentially you need to teach your body that you are going to be lifting heavier and heavier weights on a regular basis, so it’ll have to respond with more muscle to handle the load. Simple.
Eat A Calorie Surplus. To fuel the growth of your new muscle mass you’ll need to be consuming a caloric surplus – ie enough calories to not only maintain your current body weight but also to power all that extra training and muscle growth.
Get Rest. You don’t get bigger in the gym; all working out does is create the damage your body needs to respond to. So, adequate rest is crucial in order to gain muscle mass. In that sense, gaining muscle is as much about what you do outside of the gym as inside.