Forming the correct posture at the start position is a critical stage of the address position. Poor posture renders an athletic pivot motion virtually impossible.
The result of an incorrect pivot motion is the inability to swing the hands and arms in a manner that keeps the club on the correct path and the club face square through impact. Although I have made this point before, it bears repeating.
The address position is the foundation for the swing. Overlook the address position, and the rest of the swing is either impossible or just more difficult. The golf swing is a series of movements linked to one another, and each of the movements in this chain reaction are a result of the previous action.
The good news is that forming the correct posture is not difficult. Grip the club and hold it above the ground at waist high and level to the ground. Bend forward at the hips keeping the back straight and let the arms hang down so that the club is positioned correctly on the ground. Break the knees out of the locked position.
To check your posture, use a mirror. See that the back is straight, the knees are slightly flexed and the hands are hanging underneath the line of the front of the shoulders. The chin should be above shoulder level. Look down your nose at the ball. Practice this and the rest of your swing is made easier.