Dry Fire
There is a lot of misinformation on how to use an RDS, align it, and practice until it works for you. Here is what worked for me.
First, your dot must be properly aligned with your (now) backup iron sights. Proper alignment of the dot is not above the front sight, unless all you do is target shooting, not self defense.
Self defense handguns have the iron sights aligned so the Point of Impact is directly behind the front sight. Your front sight Point of Aim actually covers the exact spot where you want the bullet to go. Self defense guns are set up this way from the factory. This is one reason so many people shoot low with their new self defense gun. They are holding low. This is why the front sight "mantra" of focusing on the front sight, covering the target with the front sight and pressing, works.
For target shooting, your PoA is just below the target so you can see it (6 o'clock hold). PoI is just above the front sight.
This is different than self defense shooting. If you sight in your red dot above the front sight, then try to use it for fast defensive shooting with iron sights that put the bullet immediately behind wherever the front sight is, you will shoot high using the red dot only.
The red dot will not be any more accurate (i.e., nice, tight target groups) than your iron sights. It may be a little worse. It is faster and much more forgiving than your iron sights (think critical fractions of seconds in self defense shooting) in getting hits on target because the dot alignment does not have to be as precise as iron sight alignment. Therefore, it is for most of us more effective for the type of shooting we are practicing. Dot visible anywhere in the window AND on target, press = hit. When iron sights are not aligned, you don't know how much they are off, a lot or a little. The red dot makes that easier by giving you a significantly larger window of alignment. Also, you can shoot with both eyes open focusing on only one visual plane--the target--and not have to balance the competing interests of pistol sights and the target. When you become good at pointing, your mind will actually easily put the dot where your eyes are focused. The first time this happens, that is, you didn't have to "aim", you just point and the red dot shows up where you are focused, you will say,"SHXT!" THIS WORKS!" See the target; see the dot. Practice will get you there.
As well, much close range self defense shooting is point shooting without using the sights--simply because they are unnecessary at close ranges. However, because your RDS window is so much larger than the visual tunnel you would have to use to align iron sights, you will often be able to use the RDS window as a close range sight, and eventually you will see the dot in there too.
In self defense shooting tight groups are actually counterproductive; a quick burst of 4-5 well aimed shots spread over a 5" area (whether round or vertical), such as the high chest or head, are arguably more effective than two slower, well placed shots on top of each other. Speed is life. Slow to shoot may mean you get hit first. The larger window is especially helpful shooting while moving, when consistent iron sight alignment is next to impossible.
So, get the RDS window on your target. See the dot. Press.
All this can be practiced at home using dry fire so don't waste ammo learning at the range.
Following all dry fire rules, do 25 presentations on various targets around your place. Ignore the window and the dot at first and just use your iron sights as you always do. If you have the dot directly behind the front sight with perfect sight alignment, you won't see it anyway, but you can even have it turned off. You have to adapt your various grip techniques to get that pistol presented the same way every time. After enough presentations over a number of days, the pistol will immediately point "naturally" with your iron sights close to alignment. Remember, the front sight covers your intended PoI.
Now, if you were going to proceed with shooting just your irons (no RDS), continue dry fire until proper sight and target alignment are precise, BUT, with a red dot, you skip this step.
With natural pointing, your RDS window will be on target. Now find the dot in the window and press wherever the dot is, ignoring your iron sights. You will hit at self defense ranges. If you can't see the dot, your alignment is definitely off enough that you will likely miss. Iron sights would be at least as bad.
The beauty of the RDS is, if in live fire you fire a fast round on paper using only the RDS and the dot was not aligned with the irons, but say toward the upper right of the screen, and you get your hit, STOP. Take a breath, relax. Use your irons to point directly at your previous RDS hit, cover it with your front sight (red dot will not be visible because it is right behind your front sight) and press. Two shots together. But which was faster and simpler? Ahhhhh, the advantage of the red dot. And this exercise confirms that wherever the red dot is, that is where the bullet will go. Precise irons alignment should do the exact same thing, just slower and with more effort. Keep the irons: they are now accurate backups for you.
Dry fire. Dry Fire. Dry Fire.