extend your arm, now focus your vision on a tip of your finer, and (still focusing on a finger tip!) try to point your finger just below some small object in the distance of few meters. If you use both your eyes, if done properly it should be impossible because the object you try to point to, but not look at, will be seen double and will move as you move your finger. Then do do same but with one eye closed - now it still should be blurry, but nothing more so you can aim. This is how you aim in shooting (always focusing vision on pistol, never on target) so you need to do it with one eye. You don’t want to keep it shout as this creates additional tension in body, and probably may affect your face after years of training, so simple piece of paper or plastic to shield the eye is enough.