Constructive feedback is essential to accelerate learning progress, most customers will also need lots of encouragement.
You can think of verbal feedback as simply replaying an audio or video tape, for example,
"That was good because you checked you mirror before slowing down and, as a result, allowed plenty of time for the following driver to react to your brake-lights".
This simply re-runs the situation with a verbal description.
Feedback can relate to things have been done well or to problems,
When giving feedback about problems focus on the facts and avoid 'blaming' the learner, after all, it's unlikely that they got things wrong on purpose!
Make your feedback specific, It is not enough to say "That was very good". Although this is encouraging it doesn't give the learner any information about how to repeat the 'good' action. Explain, specifically, what was good and how they did it so that they know what to repeat (or change) next time,
"That was very good, you used the door mirror well before deciding to pull into the right-hand lane. Well done."
Next: The feedback method...