Credits go to The Review Lounge, Too at ffnet for teaching me so much about the art of reviewing. I also recycle some parts from the forum's guide for this.

Writing reviews isn't as easy as it seems, so I'll be sharing my experience.

GOOD QUALITY REVIEWS

What are the criteria that must be fulfilled so that your reviews can be said as "quality?"

1. They should contain clear evidence that you did in fact read the story you are reviewing. Generic statements that could be copied and pasted onto any story such as "good job" or "nice work" and the like aren't helpful.

At times, people think they can get away with this comment: "This story is so gory. I love it." The answer? Wrong.  You can leave that comment on any gory stories. However, it's different if you say, "Eek! Brandon bites off a dog's head? That IS so gory. But strangely, I love it." Can't possibly copy paste this comment on other stories.

There is a reason why, in my review, I prefer tackling plot and characterization. Those two aspects are what that make a story unique, and by commenting on them, it's clear that you read the story.

2. Use correct spelling and grammar. Obviously, it's okay to be a little silly sometimes, but it's also important to make yourself understood.

3. Are polite and friendly. There is something we call "tact" in reviewing. The easiest way to determine whether you're polite enough or not is by asking yourself, "If I get this kind of review, how will I feel?" However, this may not work for those who don't mind harsh review.

The most important thing in writing good reviews is actually the proof that you've read the story. Even if you can't leave concrits because you aren't so pro at writing, as long as you can prove the author that you've read the story in the review, that's already a pass.

TIPS

Many people have told me that they don't often leave reviews because they don't know what to say, they don't even know where to begin. I'm not the best reviewer in the world, but I do have some advice for those who are very unsure how to even start writing a review.

1. Find something specific in the story that you like/enjoy/appreciate and mention it. Explain why you find yourself liking it.

2. If a line or a scene stood out to you as especially good, mention that. Explain why it's so good.

3. If you see something that you think could be improved, mention it (politely). You don't have to suggest a way to fix the problem, but if you can think of a way that you think would work, the author would probably appreciate your idea even if they don't end up using it.

4. If the story made you feel happy/sad/angry/giggly, mention that. Even if it's not the emotional reaction the author was intending, it is helpful for the authors to know how the story affected you.

5. Fandom blindness is not an excuse for not being able to comment on characters. OOC is not the only thing you can gauge when talking about characters. Treat the fanfic you're reading as an original work, and the characters as OCs. How do they come across as a character? Do they stand out? Mention the traits and explain why. This can actually help the author determine whether the characters are OOC or not because if the traits you mention don't fit the author's intention, that means the author has made the character OOC.

REVIEW STYLE

1. Structured/Organized
This is the easiest style and highly recommended for beginners. See how people review anime and manga? Some reviews include categories: Plot, Characters, Sound, Art/Graphic, Enjoyment. Reviewing a story with structured style is no different, except that the Sound and Art/Graphic are replaced with Writing, in which you comment about the grammar and the writing style. Scores are optional.

Well, that's the basic. The most important thing in structured review is picking a few aspects of a story/chapter then comment on them.

Aspects list:

Opening: Does it make you want to read on? Does it lack a hook? How so?
Plot*: Clichéd but well done? Interesting idea but poor execution? Does it move too fast or too slow? What about the world-building?
Characters*: Not enough to give an impression? Feel sympathetic? Realistic? Do you find yourself rooting for character X? How so? Do mention the character's traits if possible.
Writing: Is there enough description? Too little? Too much? Does it contribute to the story's atmosphere and make you feel immersed to the story? Is the metaphor fitting or out of place? Too many adverbs? Any awkward sentences? Any info dump? How so?
Dialogue: Is it too stiff? Does it fit the age of the characters who are speaking? Does it flow well? How so?
Spelling and Grammar (SPAG): Typos? Something grammatically wrong? Do provide the correction and/or the explanation here.
Enjoyment: Does it make you laugh so hard? Cry? Give you so much feels? Creep you out? Or maybe the suspense keeps you on the edge of your seat until the end? How so?
Fandom Blind Readability**: As a fandom blind reader, can you understand the fanfic easily? Without needing to check the wiki? Does fandom blindness take away the enjoyment?
Ending: Does it make you want to proceed to the next chapter? Does you feel closure? Does it make you feel happy or sad? Is it brilliantly ambiguous? How so?

*Most important aspects. Focus on them.
**Only applicable when reviewing fanfics.

Example (My review to chapter 3 of Just Wanted You to Know, a Red Dwarf fic, by Feball3001)

Hi, Fee! Back again!

Opening

Must say it's a great one. The way it's written really gives out the melancholic atmosphere. Rimmer's so depressed with his family, because his family doesn't love him. But of course, we'll learn the truth in the funeral he's attending, since he'll be meeting his family.

Plot/Character

Yes, merging these into one since the story, in entirety, is primarily driven by Rimmer's character.
What has me intrigued here is why everybody hates Rimmer. Such as Rimmer's problem with the captain. Is it really Rimmer's fault (Rimmer isn't hallucinating)? Of course, only time will tell.
Rimmer is being so bitter in the funeral. It shows how much he hates his family. But I think him telling Karen about how Elaine isn't a faithful wife is really, really harsh, although it seems to be true (as stated by Sandra).
Rimmer's motive of going to the funeral further emphasizes his hatred. He comes not to mourn; instead, he comes to make sure that his father is really dead.
The emancipation talk is a really intriguing one. It seems more like his mother isn't that bad (except for her habit of sleeping around). She still cares for Rimmer, since she can still cry whenever she hears someone mentions Rimmer's name. Later on, her love to Rimmer also shows when she looks after Rimmer after Rimmer passes out.
Speaking of Rimmer passing out, I think it's a bit random. I mean, the cause is unclear. Rimmer says that nobody loves him, not even his friends and family. Why does he pass out at the mention of his friends' death?

Writing

Since there are many characters here and they are speaking, I feel lost at times. Like during the dialogue in the third scene, I can't keep track of who's speaking. I think by giving a break via describing other characters' facial expressions and gestures can alleviate the confusion here.
Also, I notice that the names "Arnold" and "Rimmer" are interchanged throughout the story. Because they're referring to the same person, I'd suggest just sticking to one in the narrative. It's fine if you still use "Arnold" in the dialogue, though.

Ending

What intrigues me here is how everybody behaves around Rimmer. It seems like not everybody hates him. John offers him a free house, while Sandra sympathizes with the death of Rimmer's friends. I wonder if Rimmer will warm up to everybody and eventually find out that he's never unloved.

Well done!

2. Rambly/"As I Read"
You comment on things as you read the story. Usually, this is done per scene.

Example (My review to chapter 2 of Enigma, a crossover of Persona series and Assassin Creed, by Ominae):

Hi, Ominae! Been a while since I read this, so I scanned the first chapter to remind me of what's happening. Still double fandom blind.

Okay, after a brief conversation, we're into a flashback that reveals who Daniel has actually encountered, which Daniel fails to talk about in the previous chapter. Stylistic-wise, the quick and minimum amount of narrative works pretty good there. It speeds up the pace and gets directly to the meat of this chapter, which just gets my attention quickly.

On a sidenote: on the first chapter, Daniel's name is used. On this chapter, though, Cross/Daniel's surname is used. Best to stick with just one throughout the story.

I believe that the opening speech of the flashback scene is Daniel narrating what's happening when he meets the Persona. So, I'm sure that it'll look better if you just put it at the first scene, right before the line break.

Another stylistic thing: I don't think you need to bold the speech at some places. Just stick with the italics so that they'll look better for the eyes.

Aha! It turns out that Daniel has encountered the Persona. Now, it makes me wonder if the Persona is a friend or a foe. However, seeing that they attack Daniel's crew and how they oppose the Abstergo Industries (where Daniel and co belong to), it seems like they're likely enemies. But only time will tell for sure, and I'm certain that this is more likely a story with gray vs gray morality.

The action scene during the flashback is pretty good. The imagery is clear (love the exploding eighth notes there), and it's expressed with concise word choices. During the blue halo and the loudspeaker part, I'd suggest making everything in active voice, though. Active voice is the key to make a suspenseful action/fight scene.

Heh. Too bad nobody seems to think of Daniel's news as something serious. :P At least, not until when things end.

Well done.

SOME BASIC STRUCTURES OF REVIEW

1. Opening/Intro
Of course, you need to greet the author first as a sign of courtesy. Keep it short. Because this book club is focused on fanfics, you're welcome to put fandom blind disclaimers here.

2. Content
Have you checked the sample reviews above? Notice that I tend to slip the concrit in the middle? Yes, this is what we call "sandwich." For many, receiving concrits isn't pleasant, so it's recommended to start with the good before giving concrits. It's to soften the blow.

Recently, I've tried putting concrit at the bottom, right before the ending. It also works.

3. Ending
Usually to give the authors some encouragement. I tend to just close the review with a "Well done," but this is up to you.

PRACTICING YOUR REVIEW SKILLS

The most important thing in this part is reviewing a lot. Yes, a lot. It doesn't matter whether they're fanfics (whether you know the source material or not), original works, or even just story excerpts; as long as it's writing, go ahead.

A few tips:
1. If you've just started, set the minimum goals. Set the minimum amount of words (aim around 125 words if you're really new). If you're doing structured/organized review, set the minimum amount of aspects you'll comment on.

2. Increase the minimum words. If you've been aiming to do 125 words for some time, raise to 150 or 180. Raise them gradually. The same goes to the amount of aspects in structured/organized reviews. Also, you can start aiming for Pass at this point.

3. Opening and Ending on the basic review structure doesn't count towards the word count. Neither do the lines directly taken from the stories count.