“If you choose just one goal for all your students, it would rightly be to make sure the top leg of the loop is very straight; that is most important. Concentrating on that one thing makes instruction and learning much easier. Once they accomplish that, there is rarely anything else to work on but more distance and specialty casts.”
Bruce Richards and Dusty Sprague
Some more interesting ideas can be found in the article Notes on Teaching Fly Casting by Bruce Richards and Dusty Sprague:
The first point in that piece has brought me some mildly embarrassing memories:
A couple of years ago I gave a Spey casting course to two anglers that were preparing a salmon fishing trip abroad and wanted to up their game with the double handed rod. I knew one of them from a previous course; he fly fished for trout and salmon. About fifteen minutes into the lesson I noticed that something wasn’t going properly judging by the face expression of the second student.
—Just one question —I said—, do you fly fish for trout?
—Yes —he replied—, but I use spinning tackle.
So I had been talking to him about loops —and all that stuff exclusively related to fly fishing with a fly line— without checking first if he knew the terminology… which he, obviously, didn’t!