All of these putting fundamentals are important as a group, if one of them is not followed, it will make it harder to make the putt and you will have to adjust one of the other fundamentals to compensate.
1)Eyes Over Ball
Your eyes should be directly over the ball so you can see the line from your ball to the target. If your eyes are not over the ball, your putting line will be left or right of what you think.
The intended target line should be parallel to your body line or where your feet are pointing. If you point your feet at the target, your putter face will not point at the target.
3) Putter Face
Your putter face has to point on a 90 degree angle to your intended target.
There is no perfect putting grip, if it feels good to you and the ball goes on the intended target line, keep the grip.
Basic stroke is as follows, take the putter back keeping the head low to the ground, hit the ball and follow through to the target. Keep the triangle (hands, arms, shoulders) intact, don’t bend the wrists. Speed should the same back and forth.