Find your dominant eye
Make a small diamond with both hands, keep both eyes open, and bring it back to your face. It lands on your dominant eye. Confirm by closing one eye at a time. You aim iron sights with the dominant eye.
Sight alignment
Level the front sight inside the rear notch, equal height and equal light on both sides. If the front sight rides high in the notch, the muzzle is pointed high. Line the front sight up in the rear notch, every time.
Sight picture
That is how the aligned sights sit on the target. You have three focal planes: rear sight, front sight, target. Focus on the front sight so it is crystal clear; the rear sight and target go slightly blurry. The sights tell you where the bullet is going, so trust them instead of looking over the gun to peek at the target.
New shooters rush to see the hit and push the gun down right before it fires. Find the front sight, follow through, find it again. Do not shoot and look.
Flash sight picture
From Jeff Cooper of Gunsite. Up close you only need a glimpse of the sights, as little as a twentieth of a second, to make a solid hit. As long as the front sight is roughly in the rear window and roughly on target and you press the trigger properly, the hit lands. Perfect alignment is not required at 3 to 10 yards.
Sights and trigger, trigger wins
The two big fundamentals are sights and trigger, and trigger is the more important of the two. We are visual, so we over-focus on the sights and then yank the trigger. Rough sights plus a proper press still gets the hit.
About dots
A red dot is one focal plane: the target. See the dot on the target; do not hunt for it. Dots are mostly parallax-free, so wherever the dot sits in the window, the bullet goes there with a proper press. Great for new shooters, and you can call your shots by watching the dot lift.