Be honest, but be kind. Pay attention to why the work is being written, who the audience is. Focus on comments specific to enhancing that. Read the room, the writer. If the writer is becoming upset by what you’re saying, stop. Acknowledge that writing is hard and frustrating at times. Think deeply when providing feedback.
Follow boundaries. If asked for specific critiques or things to comment on, follow them.
Ask what type of editing the author wants (developmental, copy, line by line) Find the good, comment on what you like about the writing, in addition to the areas of improvement.
Read the entire work and be honest if you haven’t had time or only skimmed it.
Give examples of sentences to substitute that would make the writing better (showing not telling; active sentences not passive sentence; or places where he pronouns are confusing). Make sure to give concrete examples. “I really like this dialogue because...Or do you think a scene should be added?
Be descriptive not prescriptive. Point out problems, don’t try to fix them - the author knows their work best.
If the writing needs a lot of help, focus on one or two themes and offer to send a resource. Ex. “Your punctuation around dialogue isn’t correct. It’s a tricky grammar topic, can I send you a link that’ll explain it better than I can?”
Learn from the work you are editing. Focus on a few things the author does that you can learn from and take the time to acknowledge that
Comment on your reactions. What are you feeling as the reader? Do you like a character, hate a character or are you bored?
Consider your relationship when you end the critique. Can you set up a phone call to discuss things further? Or is this a new author and you're unsure how they will react to your critique?