The Rule of Discipline
If they follow this rule, they will learn faster than anything else you can do. Basically this rule says "don't shoot shots you know are wrong." Here it is in all of its glory:
If anything, anything at all-mental or physical-intrudes from a prior step or from the environment, you must let down and start over.
By following this rule they will train your subconscious mind to monitor their shooting and "urge" you to letdown when something is not right, even if you do not notice it yourself! While they are shooting, they are shooting by "feel." Their eyes are focused on their sight or the target and not upon themselves. If their conscious mind doesn't focus on what they are doing "now" the shot will be bad, so all they have is how the shot feels at the time. With repetition, they can learn how a good shot feels and use that as a guide to make this call, but the word "feel" is almost the total opposite of conscious thinking, so we use our subconscious minds to monitor those.
We train our subconscious minds what to pay attention to by paying attention to it consciously while practicing . . . and that is exhausting. If you think about how you learned to tie your shoes or ride a bike, it was really hard at first but now it is almost effortless in comparison. That is because you now do these tasks subconsciously.
There has been some misunderstanding regarding the Rule of Discipline. It is not if the coach sees anything wrong, they must let down, but whether the archer thinks there is anything wrong. Beginning archers wouldn't be able to get off a shot if they had to do it perfectly in the eyes of a coach. What constitutes "right" and "wrong" for an archer evolves as they gain understanding and develop the feel of their shot and the Rule of Discipline helps them develop that "feel" and understanding. . . .
For the Rule of Discipline . . . have them use it. Every single shot. If they don't know how to do a "let down," demonstrate it for them (note the differences for indoors and out) and have them practice it a couple of times and they will be an expert in no time.