"Pistol-shooting-chart (Training aid)- How I found out I was tightening my grip while pulling the trigger. Tight groups but always pulled that direction. After using the chart, I could feel what I was doing and corrected it!" this is helpful!
Draw a circle with a square - Tack two nails to set the diameter you want, then rotate a framing square against the nails while you hold a pencil in the corner of the square. Rub a little wax on the square so it slides easily.