Having a fixed visual reference is very important for balance. Normally it's the horizon but over a golf ball the sea of grass is as disorienting as flying in the middle of a cloud.

It might seem logical to use the ball as a reference point but it disappears mid-swing and the eyes tend to reflexively track it and jerk the head around until you train them not to.

The better approach is to find some spot under the ball or just over it on your sight line to fixate on. When practicing this I stick a tee in the ground just outside the ball even with it's middle of the ball on my left-eye dominant sight line as my visual waypoint.

If you do that then take full practice swings you'll start to see the target and ground stay sharp and the club head path and ball flight as a blur. That's a sign of progress because if the ground is a blur and club and ball sharp it means you are moving your eyes and head.

Due to the way hand-eye coordination and balance work you can move the head around and if you don't lose your fix on that waypoint (tee in practice) your reflexes will bring your back to the correct position at impact. If on the other hand you try to keep the head perfectly still in space you'll wind up with a less fluid swing and compensations like a curved spine to keep the head still.

Like every other aspect of golf you might need to hit 1,000 shots with the tee outside the ball in the ground before you can do the same thing with a blade of grass or some other object. That's because the natural reflex of seeking out the horizon and fixating on ball in it's absence are so strong and ingrained.

Practice swings are a vital part of the process because it's in the practices swings that you switch mental gears so you can focus on the way point and see the ground in sharp focus as the club whizzes in a blur across the feet. With practice you'll start seeing the moment the ball disappears and be able to judge whether or not it left the face as planned from it's blur trail.