This class discussion was mostly a traditional-style lecture in which practiced the error detection method as detailed on this handout:
Part-Writing Error Checklist and Guide
When we discuss voice-leading, we are talking about melodic intervals, the horizonatal aspect of music. And because all part-writing errors are symptoms of poor voice-leading, this method finds part-writing errors by systematically comparing at the melodic intervals within each voice. It can be difficult at first to understand the method when only reading about it, but the class had no difficulty once they saw it demonstrated as discussed in the previous page (Examples 10c).
If you are struggling to implement this method while only looking at the handout, I encourage to go through the walkthrough in Examples 10c to practice finding the individual errors before working larger examples.