Fly casting is a good deal like golfing. Well, at least in the sense of the importance of the strokes (single unbroken movement) each sport uses. And for each, anyone observing the fly caster or golfer judges their competence by the way they make those strokes.
For fly casting, there are only two real strokes – the backcast that sets up the loading of the fly rod, and the forward cast that delivers the fly to the targeted area. Now, there are variations upon variations of those two strokes in terms of angles, distance to the targeted area or fish, and so on. But breaking it down to fundamentals there are only those two strokes.
And it is in the fundamentals that most beginning fly casters have difficulty.
The secret of good fly casting is that during the stroke the fly rod tip moves in a single plane – doesn’t matter if vertical, horizontal, or somewhere in between.
John Juracek is a fly fishing instructor, guide, and photographer who lives in West Yellowstone, Montana. He has written an article talking about what he sees as the failure of most new fly casters (and quite a few experienced ones) who fail to deliver the fly accurately to where intended. His well-written article can be viewed here.
I’ll only add one tip that helped me. I started with horizontal casts and broke the fly cast into two. Back cast and let the line and fly drop; then forward cast and do the same. It took only a few casts to realize how my inadequate back cast was curving off to one side. Addressing that improved everything.