The first time the trainee practices the instructor may have to do quite a bit of guidance. Having said that there should not be too much and the guidance should be limited to key points. It should be remembered at this stage that the trainees do not need to be taught anything covered in previous lessons.
If the trainee makes errors during this stage the instructor should correct them but at the same time should encourage the trainee to use logical deduction in order to work out what the errors were.
Rather than saying something along the lines of "you didn't look behind" the the instructor should say "what did you forget?" and then prompt the trainee until they remember. The theory is that people learn better whilst working it out for themselves.