Rather than focusing strictly on gaining “X” pounds of muscle—which may or may not be doable in a given period of time—work on getting stronger. Increasing strength improves your body’s ability to recruit muscle fibers, particularly the ones that make the biggest difference in the way your physique looks.
Training for strength also makes your goals more tangible and concrete. If you shoot to hit certain numbers on your lifts and then meet them, you’ll see your muscles respond along the way. Choose three exercises you want to see improvement on: one upper-body push (such as the bench press), one upper-body pull (like the chinup), and one lower-body exercise (try the deadlift). Then get to work.