Constructive Criticism. It's one word away from "Criticism", yet the two are almost completely different. Dictionary.com defines "Criticism" as:

crit⋅i⋅cism
–noun [criticism] -noun, collective [critique]
1. the act of passing judgment as to the merits of anything.
2. the act of passing severe judgment; censure; faultfinding.

In other words, criticism is finding fault in one's work. So any of the below could count as critique:

. OMG DIS SUX !!!!!!!!!1111!
. This picture is ugly.
. The hand looks awkward.

Hey, wait a moment! That third one is Constructive Criticism because it tells you what's messed up! WRONG. Constructive Criticism is when you point out errors with a piece, AND (this is the vital part) suggest/show how to fix them. So therefore the following could be counted as constructive critique:

.The hand looks a little awkward, may I suggest using a reference photo? The problem here is that it's bent back a little far than in reality it would go.
.The foot makes the person look like they are floating, can I suggest showing some shadow on the ground & exaggerating the muscles a bit to show the strain of standing up like that?
. Good job, but the eye is rather large compared to the other one. I've redlined this for you here [link].

Okay, so now you have a rough idea what Constructive Criticism (or CC) is. But I've mentioned redlining. This really only applies to drawing/painting, however it can still be very helpful.

"Redlining" [verb] To Redline, he redlined it, she is redlining.
To draw lines on a picture or piece of artwork to show how it could be improved and the correct shapes/form of the various subjects in it.

Some examples of redlining can be found across a lot of art sites, but if you search "Redlines" on dA you should get a good idea.

Anyways, back to the subject at hand, how to constructively criticise a piece. The first thing you need to do is differenciate between CC and plain critique. The key difference is that you can explain CLEARLY how the piece can be improved. For example, this:

This is okay but there are lots of things wrong.

Is critique, whereas this:

This is okay, but there are some things wrong. The eye is a bit too long (an eye should be a fifth the width of a head), the neck is a bit long (the neck is about half a head to three quarters a head long, depending on the person, and their body is too small in comparison to their head. Here is my redline [link].

Is very much CC. It may take longer to type and explain, but at the end of the day isn't it very much worth it, helping someone artistically grow? If you "don't have enough time," you shouldn't be commenting or browsing dA in the first place.

And now for what this article is about, the do's and don'ts.

DO:
- Be polite. Your comment may be very helpful but the person is not going to bother taking in anything that somebody completely rude and harsh spat out.
- Mention something you like. Even constructive criticism can get someone down if it makes everything look terrible.
- If you can't explain how to fix the picture, either redline it or link some photos/pictures to use as a reference. What is the point of CC if nobody can tell what you mean?
- Read through what you've said. A neatly disguised typo can make you look like a total quack. I remember that one time I typed; "The thumb should be placed a little lower down on the foot."

DON'T
- Don't assume that you are the holy knowledge of all art when really you may not be any better than the person you are critiquing. Always refer to anatomy references to make sure what you're saying is right, otherwise you're not correcting them, you're just getting them to draw it differently, yet incorrectly AGAIN.
- Be rude. If you got a comment like, "No offense, but this really is pretty poor, I mean come on, everything is wrong. Here's whats wrong:" because the person will most likely bin it.
- The same goes for typing properly, it's a widely known fact people will not take you seriously if U tYpE lIeK dIs !!!!!!!!!

I hope this guide has helped you out, if you have any questions, feel free to ask me them~