Thoughts on mentoring
There are probably as many styles of Mentoring as there are people who are doing it. The important thing is to find which one you are happy working with. There is no single correct method or perfect mentoring message. Think about the sorts of messages and level of detail that you would have liked to have received as a beginner and use that as a guide. Every beginner will react differently to different sorts of feedback, so don't worry if your style seems different to how others describe theirs, it might be just exactly the style that your beginner will respond to best.
Feedback is a part of Mentoring, but not all of it. Feedback is appropriate on any book. It can come from P2 to P1, from F2 to F1, from a post-processor to a proofer from any round, or from a PG white-washer or post-processing verifier to a post-processor. Feedback is "you did X and this is what I thought of it".
Mentoring also means welcoming and encouraging a beginner, not being negative. It means directing them to the tools so they can become more self-sufficient. It means suggesting practices that will lead to more success and enjoyment. Knowledge of these resources and practices evolves over time; these resources and practices are not yet used well by beginners.
It can help to take note of the corrections you have to apply as well as making the fixes. Take note of things that the beginner had done properly as well. Look for consistencies (both errors and things done correctly) and try to understand what the beginner did and didn't "get". (Had they missed a Guideline section, missed the whole Guideline document, gotten confused about a difficult section? ... diagnose what is happening.)
When composing feedback messages, the primary goal is encouragement. If they're trying, but missing several things... you may only bring up the most egregious. If they're doing great work, encourage them to try more difficult texts. Always try and include at least one positive point; even if they only corrected a single error out of hundreds on the page, they have made it better, which, in the end, is the goal of P1.
You may want to save the messages you compose into a "feedback template". You'll find yourself writing the same sort of messages and quoting the same sections of the guidelines to many proofers. Build your own template, to your own unique style, and add to it as you address different issues with different new proofers. It will help take some of the repetition out of writing feedback, and leave you with more of the satisfaction of helping a new proofer get a good start.