The two major causes of fat and thin shots are poor weight distribution and early release of the hands. Here I'll show how to take care of both.
Photo 1 shows that my weight is behind the ball line and the club face is now ahead of the hands. One or both of these will cause the fat or thin shot. Notice that the center of the hips and sternum are behind the ball. To eliminate this, you must have at least 70 to 80 percent of weight on the forward side at impact, as in Photo 2. The hands should also be slightly ahead of the clubface with some shaft lean.
You will also notice that with the weight and hands forward, the center of the hips are now past the ball and the sternum is in line with or just slightly ahead of the ball. This will produce more of a downward strike, creating better contact, power and distance.
From the top of the swing, start down by driving your right hip down toward the ball, as I am in Photo 3. This will move your weight left and through the ball. At the same time swing your arms down, keeping your forearms close together, and swinging through the ball with the handle leading the club face. This will help you to develop a late release of the clubhead.