Trigger pressure must come from the trigger finger only. You must place the trigger finger on the trigger in such a way that you can squeeze straight to the rear and thereby have no disturbance to the sight alignment in any way. Having the arm so solid, the wrist straight, and the grip hard will aid you in trigger control. By gripping hard you can keep the same grip on the pistol throughout the shot or series. The trigger finger must move indepently from the rest of the hand.
Consistency is the secret of doing anything well.
This is especially true of trigger control. He must learn this lesson early in his training. Even the most experienced shooters never hold the gun absolutely still, but the attempt to hold it still is always present. The experienced shooter has many shots break when the gun is almost motionless, and this is what the new shooter must be striving for in his performance. But when the gun still moves slightly he must accept this movement, always attempting to hold the weapon as still as it is possible for him to do so.