PostureSuperResource is a free web book that takes a habit-based approach to improving one’s posture. Each chapter focuses on a single habit that you can implement in your life; multiple such habits implemented in your life can nudge your posture into a more optimal posture.
The habits are synergistic; the more of them you can do the better. It is better to implement these habits concurrently (at the same time), rather than sequentially (doing one habit for one month then another the next month). The first two habits in particular work well together. HABIT#1 (Corrective Exercise Regimen) is something you can do daily during the day, while HABIT#2 (Sleeping Positions) is something you do passively during sleep. I recommend you read both of these chapters to start.
Click the link below to read about HABIT#1 on doing daily corrective exercises.