Staying on plane a matter of feel
There's only one thing that all good golfers do when they are playing really well regardless of style, size, shape, or method - they stay on plane.
When they hit bad shots or lose form altogether it is very often simply because they get off plane. They bend their swing-plane without being aware of it.
Ben Hogan pictured the swing-plane as an imaginary pane of glass resting on his shoulders with his head sticking through a hole in it. But it is more satisfactory to think of the hole being bigger and one's torso sticking through it. Then the clubshaft can stay on the surface of the glass from the start of the downswing to the end of the follow-through.
Whether one swings flat or upright doesn't matter. One just keeps the club swinging on the glass. Get the picture?
Or you may prefer to think of the swing as resembling a wheel. A tilted wheel , of course. The clubhead swings round the rim.
Now obviously you don't want to buckle the wheel. A buckled wheel is of no use. It's got to be straight.
You may like to picture your hands also describing a circle, a wheel within a wheel so to speak. Again the rim mustn't bend.
With the glass, the bottom edge (and thus the whole plane) can be aimed in whatever direction you please. With the wheel you can imagine a track running out either straight ahead or at a slight angle.
Now get your pro or a friend to stand behind you as you do some practice swings and see if your own view of whether you've buckled the plane or not agrees with theirs. When you reach the point where everyone is agreed that you have stayed on plane, treasure the feeling. Run it through again. Bank it.
But also do it wrong a few times and note that feeling too.
In golf, as in life, learn to tell wrong from right just by the feel of it.