The process of building muscle is pretty straightforward. Train the muscles in question to failure about twice a week, and they’ll grow. You can do it more often than that, but studies show it doesn’t tend to help much.
Muscles need dietary support to grow, though, and that’s where things get tricky if you’re looking for lean muscle mass. To add muscle tissue, you need to eat above your typical daily caloric needs. Of course, that’s also the recipe for adding stored body fat, which creates a conundrum; adding muscle also tends to mean taking on at least a little fat with it, or at least hanging on to what’s already there.
Much of the bodybuilding community has come to the consensus that you can’t shed stored fat while you’re in a serious muscle-building program. The solution that most take is to break up the regimen into “cutting” and “bulking” phases. Bulking is the period in which you focus on getting your muscle mass and strength to where you want it to be, without worrying so much about where your body-fat levels. The cutting period is the time to focus solely on maintaining your muscle gains to the greatest degree possible while shedding as much body fat as you can.
One can time these out in any way they wish, but most adherents to this regimen will cut in the lead-up to and during the warm months (as they’re more likely to have lighter clothing on and be showing more of their body during this time). They’ll then bulk during the colder months when they tend to be bundled up. Professional bodybuilders will time their cycles more around competitions so that they’re cutting just before performing.
Regardless of whether you’re cutting or bulking, however, the types of food you eat and supplements you take remain pretty much the same. There are varying schools of thought on this subject, but a widely agreed-upon number is that the muscle-building diet should consist of at least 50% protein. You can then divide the rest up roughly evenly between slow-digesting carbs and healthy fat (while not forgetting to get your needed vitamins, minerals, and dietary fiber in).
That brings us to the final question — how do you know how many total calories to take in? It starts by knowing your “basal metabolic rate,” which is the number of calories you need each day to maintain your current weight if you were to engage in no physical activity whatsoever. This calculator can give you a quick estimate. If you’re bulking, you’ll need to eat more than your BMR plus whatever is burned by your physical activity to add to muscle tissue. If you’re cutting, you need to go below your BMR. You can safely lose roughly 1 kilogram of fat per week without also losing significant muscle tissue.