If you want to be a writer ...
be a good reviewer
Yesterday I was talking about receiving criticism and why it's important that writers understand the difference between support and unconditional love. The former helps us improve our work, the latter helps us bear the former; writers, readers and (if we're lucky) editors give us support, and family and friends and pets give us unconditional love. Confusing the two leads to rants, sulks and appalling behaviour from writers and irritation, rejection and justifiable withdrawal from all the other parties.
So you're going to give somebody else the kind of critique that you'd like to recieve yourself. How do you go about it? Look at it this way ... what would you like a reader of your work to comment on?
My list would be:
Handling of themes
I like line edits if somebody wants to offer them, but I don't think they're obligatory. What's much more valuable is a sense of what the reader found to be strong or weak in the story; (not what they like or don't like, which isn't much help unless they're going to publish the piece!) - anything they struggled to understand; whether they thought the themes/plot/characters were original or cliched etc. What isn't helpful is when a reviewer rewrites the story saying, 'I would have done this' - suggesting enhancements is fine, but taking over somebody else's creation is not.
It's best to read work one day and comment the next - I often find that my students have much more to say about each other's work when they've had a period of time to reflect on it and read through it again to check their understanding and responses. While this isn't always possible, you can be pretty certain that you'll provide a better response to the reader if you can give yourself thinking time.
Remember that critiquing work should be as valuable to you as to the recipient. Did you find the work easy to read the first time? What images or ideas stayed with you when you'd read it? Were there clunky sections of prose, limp or predictable scenarios, cardboard characters? What was fresh? Now reflect on your own work - how many of those faults appear in your fiction? Reading and commenting with care and attention help you to eradicate those weaknesses; it's hard to spot them in our own work, much easier to spot them in the work of others, but continued exploration of the mechanics of fiction will help you to avoid them altogether - and the simplest way to learn about what works and what doesn't in fiction is to help others to improve their stories.