Critiquing another writer's work is problematic. Some writers ask for critiques but what they really want is praise. They want you to say,"Boy that's wonderful! I don't know why it got a rejection". This is what they want even though their mouths are saying "Please tell me the truth I really want to improve". They don't want the truth unless the truth is "Boy that's wonderful! I don't know why it got a rejection". I know these writers because sometimes I'm in that category, I have to admit it.
I've written something I love, I've sent it off and it was slapped down without a second thought and my feelings are hurt. I think the best thing to do then is read it to your husband or other family member who thinks you're the best thing since cheese and then let him say "Boy that's wonderful! I don't know why it got a rejection". Please don't give it to another writer and pretend like you want a critique, a real one, one that will help you to improve the story. Please don't do that- it's not fair.
Critiquing is a big job. For me critiquing another writer's fiction may be the job I like least of all. Mostly because I have trouble staying out of it. Also you don't want to hurt people's feelings, you just want to improve things a bit, but you don't know which turn of phrase they are so proud of, which character quirk they got in a flash of brilliance, which plot twist they are sure is the key to the story's success. It could be the very one you don't like. It's tricky this business.
I have improved along the way; on both sides- giving criticism and accepting criticism. Rick Daley wrote a lovely post at Nathan Bransford's blog that gives some pointers and I have a few of my own gathered along the way.
1. Use Word's tracking tool.
I used to not know about this and just made changes in brackets on the document or highlighted with colour. I think the tracking tool is less invasive, it feels more like the suggestions that the critique really is.
2. Watch your wording
Unless the person is a very close writing friend, saying things such as "Get rid of this" might not wash well. Maybe use "Might be stronger without this" you still get the point across but again as a suggestion not an order.
3. If things are in a bit of a mess, don't nitpick with grammar
I know people may disagree with this but I feel if the plot and characters are not working you rather leave grammar issues for the time being. Too much colour on the returned manuscript is demoralising for anyone.
4. Don't miss the good stuff.
Don't only comment on what's wrong. Highlight also what is right. Mention that beautiful wording, dialogue that is right on base, a refreshing ending.
5. Give a positive round-up.
I always try to end with my general impression of the piece; what worked, what didn't. I try always to be positive but I'm never dishonest. If it is not working it's not working and I won't say it is. In a true critique, you're doing no one a favour by giving false praise. Don't be negative but don't be dishonest.