I started a topic about this some time ago, just spouting my own opinions:
I never summarized it in a readable format, so here we go: 
This is for offline reviews, meaning: when you’re not having a dialog with the student at the same time.
The aim of a good review is to identify the student’s worst mistake and build one effective lesson around it.
To do this, I look over the whole game to see where that worst mistake is. The mistake is not the move itself; it is just the symptom. Try to comprehend where they are coming from. There is an idea in the student’s head that made them play those moves. Focus on this as the spanning theme of the review.
Next, use the following ingredients to construct an effective lesson:
- Identify the concrete mistake and the erroneous thought process behind it. Put it into words and make it clear to them.
- Offer positive advice. Tell them what to do, not only what not to do. Our brains don’t pick up on don’ts.
- Justify everything from general concepts and proverbs, not just variations. The student must learn to apply this knowledge in a new situation.
- Engage (a little bit). For example, ask the student what the next big move is, immediately before you suggest it as a variation.
- Use clear language and talk in a way that will “get to them”. You need to try and connect with the student on an engaging level. They should read your comments and take them to heart, instead of falling asleep or rolling their eyes
Some examples:
“If you had done this one thing better, you might not have lost.” encourages the student to want to learn the one thing and apply it.
I usually leave a summary comment at the end, addressed to “Dear Student,” followed by the pros and cons of their play and some suggestions on how to improve. It’s a great place to establish the spanning theme (like “aggression”), because it is the last piece of information and will thus be remembered better.
Negative example: never leave a comment on a move like “this is bad because you are pushing from behind.” → Student’s face: “wtf, what else am I supposed to do then?!?” - Tell them where to play instead.
That can get a bit tricky when you get to unnecessary exchanges (which should be left out). I usually turn it into “be careful” and “make purposeful moves”.
When you say that B is better than A, you always need to give a justification. For example, “A makes you heavy, B is light, and you are willing to sacrifice the marked stones”. Use concepts like “thickness” and proverbs to connect the idea with the situation on the board in their head.
I like to use superlatives and exaggerations to get the point across, like “Black just got the biggest bestest wall ever while white lives with 2 points”. Don’t forget to mention features that “dominate the whole board” or moves that “land a knock-out blow”.
Someone asked me for a review recently. My reply was (as usual): “Get back to me with one loss that you played to the best of your ability”. Several days later, I heard back from them. They thanked me and said that they had no loss to show me, but had improved and ranked up thanks to their radical shift in attitude following my challenge.
Don’t give them the lazy AI review. 
Above all should be your concern for the student’s improvement.