Just eat. A lot. More calories in than out, it's a very simple mathematical formula.
I'm 6'0, was only 137 pounds, gained about 22, so now I fluctuate around 159. I placed a heavy focus on cardio and ab workouts though in order to maintain a good level of fitness. I know large guys who are almost body builder types who can run /fast/.
Conversely, I also know guys who have me beat in terms of sheer mass and strength, but they can barely run to save their lives.
Don't have to make any drastic changes, as long as you aren't eating solidified grease, just keep eating whatever you already are, just go for additional helpings.