I think a lot of the skill in instructing comes to play in this, and experience plays a part.How do you get them from where they are to mastery? Is it just straight repetition with feedback?
1. You begin with a good briefing on the skill. Over the years I learned what the most common problems are likely to be with each skill, and I tried to head them off in the briefing.
2. When an experienced instructor watches a student struggle, the instructor can immediately see why the student is struggling and can then provide the appropriate help.
My very first scuba experience was a Discover Scuba in a pool, and I had no trouble clearing my mask from the start. Thus, when I became an instructor and saw students struggle, I had to puzzle out the issues they were having. Eventually I saw every possible mistake and finally came to an understanding of what had really come naturally to me. After that, I was able to give a far better briefing and demonstration, and my students almost always did it well the first time. If they had trouble, I was able to see why and correct it easily.
You can also head off problems before you teach the first skill. Many students have problems because they feel like they have to act too quickly on a skill. This is especially true of any skill that requires taking out the regulator. At the very beginning, before they do the first skill, I would do a demonstration in which I would lie on my side facing them, my elbow on the floor, my head supported by my hand, in the attitude of bored leisure. I would take my regulator out, showing that the mouthpiece was pointing down, and set it on the floor. I would point to the tiny stream of bubble coming from my mouth. I would then continue the bored leisure pose, drumming my fingers on the floor, glancing at my watch. Then I would put the regulator back, purge, and go on breathing. That demo had a HUGE impact on the students and their skill performances that followed.