Left Hand Grip
When gripping the club with your Left Hand (right handed players) it is very important that you first align the clubface to your target and then position your Left Hand in a "Vertical" to the ground position on the target side of the grip of the club.
Fold the Left Hand around the grip making sure that the heel pad opf your Left Hand is on top of the club and place your Left Thumb on the "Aft" side of the grip.
At first it is tempting to move the Left Hand to the right to get the Left Thumb in the desired position in line with Pressure Point #3 (first joint of the right index finger).
I over came this temptation by not positioning the Left Thumb until I had both hands on the shaft.
Then the last thing I do is open the right hand slightly and then move the Left Thumb without changing the Left Hand from its "Vertical" to the ground position.
I must admit, in the beginning it feels very weird to do this. If you try to position the left thumb as you position the Left Hand it is tempting to also move the Left Hand to the right to far and loose the "Vertical" to the ground position.
There will be a very noticeable gap between the Left Thumb and the fore finger. It will no longer be a closed "V".
At the top of your backswing, you will no longer have a feeling of having the Left Thumb "Under" the shaft supporting the club.
Never take your grip for granted. It is a Look, Look, Look, at your hands.
Homer Kelley author of "The Golfing Machine" was emphatic that the Left Thumb be positioned on the "Aft" side of the club.
The point I am making, from Homers point of view, the left thumb MUST be in line with Pessure Point #3, not on top of the shaft.
He said having the Left Thumb positioned on the top of the club did absolutley nothing to assist the shot.
For further reference refer to The Golfing Machine Book 7th Edition 7-2 and 10-2-B