This is my take on shooting accurately. I shoot mostly action disciplines so shooting fast and accurately enough is my focus. If you are doing precision slow fire, the emphasis will be slightly different.
Firstly, a piece of very important information - nobody, that is absolutely nobody can hold a gun completely still. There is always some movement. Practice can improve your abilities, but while you are still made of flesh and bone with a pulse and muscles, you will always have some movement. Try to minimize the movement that you see in the sights, but accept that there will be some.
Trigger finger placement is not the be all and end all. What you want to achieve is to be pulling straight back at the point where the trigger breaks and the shot fires.
Your finger cannot be pulling straight back for the entire trigger pull because it is rotating on 2 pivots - mostly your 2nd knuckle.
Where on your trigger finger you place the trigger will depend on the size of your hand, the length of your finger, the size of the gun - both width and length of pull.
A good place to start is with the middle of the pad of your finger centered on the trigger. You will almost certainly need to adjust depending on your observations.
With the gun unloaded - double check - and then triple check - hold the gun as if you are going to fire. Grip it tightly. The gun isn’t going to fire this time, but it will when you do this at the range so grip it like you mean it.
Align the sights aiming at some small object, maybe a light switch across the room, concentrating focus on the front sight and watching closely to maintain equal amounts of light either side. Put your finger on the trigger and slowly press it back watching for any deviation in the sight alignment while doing so. When you see movement in the sights correct it, but do not stop adding pressure to the trigger. Stopping and starting your trigger finger during the press is another way to induce inaccuracy. You need to find your sweet spot with your hand and that gun. You can experiment with different positions of your trigger finger as well as grip panels or backstrap inserts to see what works best for you.
When you get to the range, press the trigger slowly so you can see any movement in the sights as you do so. A very common error that newer shooters make is they align the sights with the target and see a good sight picture and then they press the trigger really quickly to try to grab that shot before it goes away. What happens is that they move the gun as they do this, but it is so quick and the recoil comes so soon after that they do not see the change in the sight alignment and cannot figure out why they didn’t hit where they expected.
The more important part of shooting accurately, is your grip on the gun, rather than your trigger finger placement. Hold the gun firmly in your dominant hand - as tight as you can without limiting your ability to control your trigger finger. With practice you will be able to grip firmly with 3 fingers while maintaining independent movement of your trigger finger. That is unlikely to be an ability that most people have naturally. Grip with your support hand as tightly as you can without introducing a tremor. Most people can grip with a pressure measured in 10’s of pounds. Most triggers are 6lb, many significantly less. Let’s say your grip across both hands is 30lb. Pressing with 6lb of pressure on the trigger should have little to no effect on that grip - try pulling a 30lb weight with a 5 lb spring. It isn’t going to move much.
One place that trigger finger placement can become a factor is if the part of your palm at the closest knuckle of your trigger finger pushes on the gun when you pull back on the trigger, it will push the gun away from your dominant hand. Try not to put that much finger into the trigger.
If you are having accuracy issues, it is usually worth spending some time with an experienced instructor who can diagnose what part your technique you need to work on. It is a fallacy that you “just need to practice more”. Most newer shooters are doing something to make their shots inaccurate, but don’t know what it is and it is hard to spot from directly behind the gun. A 2nd person watching can see things you cannot. Repeatedly practicing whatever poor technique that is being used will only make it harder to correct once it is identified.