The secret to bulking up muscle is simple. To bulk up muscle you need to consume a lot of nutrition and lift heavy weights. Eat all the you can during the bulk up period and lift as much weight as you can even if you can only do a few reps and require long rest periods between sets.
If you are really serious about bulking up I would even suggest to go as far as not doing any calorie burning activities such as cardio for 1 month. This technique really works out well because you can get the bulk up on muscle first then work on definition the next month.