Good feedback is priceless to a writer. To be good,
however, feedback needs to include specific
information: ANY specific information. A comment on
a character, a quote, a bit of dialog or even a
setting detail often does several things for the
writer: s/he gets a peek into the readers’ minds, to
see how s/he responds to the writing; s/he has an
opportunity to look at her own work in a new light;
and s/he has the benefit of learning new tidbits a
reader might be able to offer. A seed of a
suggestion can grow in a writer’s fertile
imagination and improve his/her writing in many
ways.
 
What specific things can you comment on? Anything
you notice. First, a good, close read of the
material is essential. After your first time
through, look away from the text for a minute or
two. Ask yourself, "What do I remember? What stood
out?" Anything that stands out from the rest of the
writing does so for a reason; it was either good
enough to make your brain smile, or bad enough to
make it blanch. 
 
For a good reaction, the specific review is easier.
Think next about why you liked a certain aspect of
the story. Was the character just like someone you
know? Did the action give you a sense of
satisfaction? Was the dialog exactly what you wish
you could say to your boss? Was a description so
vivid that you felt as if you were seeing the item
described? Your writer would like to know this! If
you read several pieces by a particular author, you
may notice that you like the same things about the
works- there is always good action, or the story
just sucked you in, or you are secretly in love with
the main character. You are noticing the strengths
of that writer. Some writers excel at writing
characters that we respond to; no matter which Dick
Francis book I pick up, I know I’ll love the main
character, who will be heroic, strong, self-effacing
and honest. Stephen King is the master of mood-
nothing scares the daylights out of me like his
stories. Some books are action oriented; those
writers excel at plot. Mystery writers are all about
detail. These professional writers know what they
are good at, and write to emphasize their talents.
Beginning writers often don’t have this information.
They depend on you to help them develop their
strengths. Tell them what they are best it. You
might mention only one detail, but if other
reviewers all mention the same thing, that writer
has valuable information about his or her writing to
build on. 
 
It shouldn’t be hard to pick out one thing to praise
in a story. Sometimes, however, there is a
submission that lacks dazzle. It just sits there. It
isn’t particularly interesting, it goes nowhere, it
didn’t have a point; whatever the problem, the
writer also needs to know this about his/her
writing. George Eliot went on for most of a page
describing a pair of hands in Silas Marner.
Description is key, but a beginning writer can go
overboard, getting bogged down in the mundane. Too
much of a good thing is usually a bad thing. Point
it out!
 
The kindest way to structure your review is to begin
with the good, then mention the disappointing, along
with a few ideas that would have helped you enjoy
the piece more. This takes careful thought on your
part, but it isn’t difficult. If the piece seems
pointless, suggest an ending (or suggest that it
have an ending, if you can’t come up with one.) If
it is dull, ask for more details about your favorite
part. If the character suddenly changes in the
middle so drastically that you suspect a demonic
possession, suggest that the characterization needs
a bit more continuity, and suggest specifically how
you’d like to see the character act instead. 
 
Writing can be tricky, and stories can sometimes
have a mind of their own. Sometimes the story gets
away from the writer before s/he is aware of it. A
good review will help the writer take back control
by pointing out problem areas, even if it is small.
If you happen to know some detail in the story is
incorrect due to a particular area of expertise you
might have, by all means make a suggestion. If
something just sounds wrong, but you have no idea
why, point out what exactly sounds wrong, and
suggest more research. If you have taken the time to
find a source, your writer will take your suggest
even more seriously and be more likely to act on it.
 

Finally, however, remember that stories, good or
bad, belong to the writer. Your review, and any
advice you offer, is just that- advice. It is
ultimately up to the writer to decide whether or not
to use your suggestions or discard your review. A
good writer will accept thoughtful review, and find
general statements like "It was great!" to be
encouraging, at times, but useless for revision or
improvement. (Same goes for "This is crap!" although
this tends to be significantly less encouraging, and
may lead a writer to give up altogether.)
 
To get started, it may help, if you tend to forget
your initial thoughts, as I do, to copy and paste a
whole section into a word document. Then, as you
read, you can comment right into the body of the
text, or add question marks to remind yourself of
passages that confuse you. These notes are handy to
have when you begin your review, and the longer the
piece, the more essential they are. 
 
Now, go find some unsuspecting author and study his
or her writing. Good luck, and happy reviewing!