A couple days ago, my posting was about the way a writer responds to being critiqued. The critiques are meant to help a writer grow, but some take them as a slap in the face and storm away, thus learning nothng.
The other side of the critiquing coin is on giving critiques. It's not something to be done carelessly. A good critique involves a lot of thought. To critique properly, you need to read the submission through several times. Then give it a little time to float around in your mind, read it again and then proceed.
It's always good to start with the positives, for we all like to know we've done something well, don't we? And it's a rare submission that doesn't have at least a few things that make it praiseworthy. Next, give an overall statement about ways that the piece might be made better, stronger, or more interesting. Give examples of what you're suggesting to help the writer get started. It's beneficial to add that these are your opinions and that the writer should take what she can use and discard the rest. For after all, a critique is definitely one person's opinion. It's not all right or all wrong.
Most critiques in my online group include a LBL, which is a Line By Line, critique within the text. The critter puts her comments in CAPS within parentheses--not to be shouting but to make her comments easier to find. She'll mark words that can be deleted, places where something added would help, or perhaps a better way to state something.
When you critque something for another writer, be fair, be honest, but also be encouraging whenever possible. Your job is to help, not tear somebody down. You might consider yourself a doctor of words.