1. The first step is increase your protein intake! Protein is the building block for larger muscles.
2. Increase the weight your using during your workouts, You will want to do less reps and more weight.
3. Do your max weight several reps by doing negatives. You will need a spotter to help you. The idea is going down very slow and having the spotter help you push it up. This will maximize growth and size by forcing the muscle fibers to breakdown and later expand.
4. Drink plenty of water! Water helps with building muscle tissue.
5. Change up your workouts and don't be afraid to go to failure.