Here is the Army Marksmanship Unit description of a proper grip:
3. Method of getting the proper grip: The proper grip must conform to all of the foregoing requirements plus it must be a hard grip and it must be adapted to the hand of the individual shooter. NOTE: FOR THIS INSTRUCTION IN OBTAINING THE PROPER GRIP, THE WEAPON IS THE .45 CALIBER SERVICE PISTOL. THE FOLLOWING STEP-BY-STEP SEQUENCE WILL PROVIDE THE PROPER GRIP: a. With the non-shooting hand, pick up the pistol by the barrel and of the slide, being careful not to mar the blackened sight and keep the muzzle pointed down range. b. Spread the index finger and thumb of the shooting hand apart to form a "V", with the thumb held slightly lower than the index finger. c. Bend the wrist slightly downward to obtain proper angle of contact. d. Fit the pistol into the "V" of the thumb and index fingers by seating the grip safety straight and firmly into the loose "web" of akin in the "V". e. Press downward on the barrel to pivot and push the mainspring housing firmly against the inside of the bulge of flesh at the base of the thumb and into the depression in the approximate center of the heel of the palm. f. Stretch the fingers forward, letting the trigger finger come to rest flat against the pistol frame just above the trigger guard. Safety dictates the trigger not be contacted at this time. g. The lower three fingers should come to rest closely touching each other, with the center bone of each finger resting on the curved front surface or "front strap" of the receiver. Little or no pressure should be exerted on the finger tips extending around the front strap to the surface of the left handgrip. Pressure exerted on the front strap by the little finger should be lighter than that brought to bear by the middle and ring fingers. Too much pressure with the little finger may cause the muzzle to depress slightly, resulting in the front sight aligning low in the rear sight notch. h. The thumb should be raised to a level higher than the index or trigger finger. Only the joint at the middle of the thumb is high against the stock in the vicinity of the slide safety. The end of the thumb is turned up and away from the stock as it has no function. Pressure exerted on the aide of the pistol by the end of the thumb has a tendency to disturb sight alignment. The thumb should be raised to a level higher than the index or trigger finger. Only the joint at the middle of the thumb is high against the stock in the vicinity of the slide safety. The end of the thumb is turned up and away from the stock as it has no function. Pressure exerted on the aide of the pistol by the end of the thumb has a tendency to disturb sight alignment. The thumb should not exert great pressure on the aide of the pistol as early fatigue will result. Only required substantial supporting force should be exerted to hold the weapon firmly in place in the shooting hand. i. A controlling grip can be affected by the three lower fingers directing primary pressure on the front strap straight to the rear, pressing the mainspring housing and grip safety firmly against the side of the center depression and the heel of the palm at the base of the thumb, and the loose flesh in the "V" of the thumb and index finger, respectively. This can be compared to a vise with the inner surfaces of the palm as the stationary jaw of the vise and the three lower fingers pressing on the front strap of the pistol as the moving jaw.