Much has been written about posture. You have to set the correct body angles at address to swing correctly on plane. Once you can do that consistently, your swing has to improve.
Some angles that are sometimes overlooked are the positions of the shoulders, knees and feet and how the arms hang.
An imaginary line should connect the top of your feet, touch your knees, and continue into your shoulders. That establishes the amount of flex you need.
An imaginary line that extends from your shoulders down to the ground clearly shows how your arms should hang naturally. Let gravity properly position the arms to set the correct shaft angle.