Here are the basics:
Train at least twice a week, preferably three times, with weights. You can do bodyweight exercises but you won’t get the gains available with external weights.
Complete 6-10 exercises per session for 10 to 15 repetitions.
You can use a split system in which you only do arms, legs, back, core etc on certain days, or you can do a full-body workout covering all the main muscles in a single session.
Eat appropriately for the workouts you do, ensuring sufficient carbohydrate for energy, and protein for muscle rebuilding and synthesis.
Get plenty of rest as muscle rebuilding and enhancement often occurs when you are recovering and during sleep when growth hormone is at its peak.
How Much Muscle Can You Build Quickly?
It’s not easy to put on a lot of muscle and there is a genetic component; some men and women do much better than others in a shorter time frame. However, for younger men, the numbers are probably in the range of 2 to 5 pounds per month as estimated by experts in muscle physiology. Twelve pounds in 12 months may be a lofty goal, but probably not impossible for some fast gainers.