I got asked this question recently and answered it with a video. Here it is:

There are two main things you can do:

• Eat like a horse
• Work out like a horse

There are more things involved, such as getting enough sleep and focusing on big, whole body exercises such as the deadlift, the squat, the clean and press and other similar exercises, but those two things, if put in practice with determination and with the clear goal in mind of getting big, will take you from a skinny ectomorph of 135 lbs. to a muscular, strong 195 lbs. See the diptych below. On the left is what I looked like when I started college and on the right is what I looked like when I graduated.

There is one more thing I must mention: there is a price to pay for this and it’s also big. Huge. You will inevitably get gas from all the protein-rich foods you have to eat frequently during the day. It’s not a toot here and there. Exactly when you make most of your gains in mass, you have to prepare for flat-out flatulence, all day long and all night long. It gets nasty. The odor is unbearable. Intolerable. It’s not like I was gulping down cheap buckets of mass gainers. I was eating healthy foods from the cafeteria at one of the top liberal arts colleges in the US. Between meals, I was drinking quality whey shakes and supplementing with cans of tuna. But it was very, very unpleasant. I loved working out, I loved getting big, I loved the way I looked and felt but boy, did I hate the farts… I messed up my gut for a number of years and I’m still not sure if my gut is fully recovered these days. I wonder if I did something wrong with my diet back then and if I were to do it again, I’d focus on gaining weight more moderately and more slowly and definitely eating less, even if it meant the progress would be much more gradual. Food for thought…

Hope you find the video helpful!