A knack for usability testing

What differentiates an awesome usability test from an average one?

As I have been mentoring UX researchers, I noticed that there are certain techniques they weren't aware of or misunderstood despite having years of experience on the job.

The twist: instead of writing a book about it, I wrote you letters. Each letter explains a specific technique, why it's important, and how it works. 65 in total. Complete with examples and illustrations.