To gain weight i need to eat a decent diet, work out hard, drink water, take in enough protein and add 300-500 extra calories a day right?

And recover: You're right; the one thing i would add is build in time for rest and recovery. Your body grows between your weight training sessions. Ideally you also get 8 hours of sleep a night, and schedule rest days in your workout schedule. Four days a week is plenty to stimulate muscle hypertrophy; more and you risk not giving yourself enough recovery time.