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 huge, 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.