Throwing out some lessons to me, maybe you have some to add.
#1 Thank the writer. It takes guts to open yourself up and say hit me with your best shot. Appreciate the human being who’s letting their guard down. Their asking for help not hurt.
#2 Like it before you open it. The writer is probably not aiming for an audience that hates what they write and picks it up anyway. The end user will be someone who looked at the cover, looked at the synopsis and said that is something I WOULD LIKE to read. Even when you’re on chapter 10 and 1-9 did not impress, the same still applies.
#3 Read frequently. A story is best when the characters/setting/plot are fresh. Especially with a longer work, odds are the writer didn’t intend you to put it down for weeks or months between readings. If you are picking up the next chapter and asking who a main character is, you probably aren’t doing the reader a service.
#4 Help them with THEIR work. If the writer wanted to know how you would do a story, their request would contain the words “You should write a story about . . . ” They are asking for a critique so that THEIR story becomes the best it can be. So don’t ask, how would I do this? Ask “How can I help them get to their goal?”
#5 Tell the truth. They want a critique so they know how to get better. That doesn’t mean be tactless. You should be able to tell someone that their work sucks without telling them “your work sucks.” If you can’t, maybe you aren’t looking at rule #2. But do tell them it sucks when it does.
#6 Because the truth you told them in #5 is your opinion, and because it’s their story (#4), what a critique is really asking is HOW YOU TOOK IT. They are not wrong for the POV they chose or the fact that their character is not or is too conventional. But they do want to know what you thought about it. A good critique is not about what they did, but about how you took it. A good critic therefore is someone who can articulate WHAT triggered their opinion. “This scene was boring” is not as helpful as “I just felt the conflict slipping away. So and so seems to be doing fine, they aren’t affected by the threat so I felt they were safe. Safe meant boring. Put something at risk!”
That’s what I’ve learned, and what I hope I am to others, and they to me.