You look over the submitted work. From what you see you then go look thru the lesson text to find the exact section that explains the errors they are making. You then link them back to that section, adding some useful comments of your own if you think that will help clarify.

For the earlier lessons specially, everything is on the lesson text. You don't really need to come up with critique as much as point them to the right lesson and section. If you know something that's not on the lesson text, then don't make it part of your critique either.

The creator has included and also omitted just the right things from lessons, and all that comes from years and years of experience giving critique to thousands of students. You shouldn't claim to know better when just starting to give out critique.

On the more technical side of critiquing homeworks, for lesson 1 and 2 in particular I really like the method where you go through the submission exercise by exercise in a sort of bulleted list.

Example:

Your superimposed lines are good, insert stuff here for detail

It looks like you might have rushed through your ghosted lines, so slow down and be sure to bla bla bla

You can see a more thorough example of this on my profile in the replies section. Makes it easier to break things apart and be clear about what exactly you're addressing. Typically I see improvement throughout the lesson, and so will edit some of the first things I said when I'm finishing up the critique.

Example:

First version- "Your ghosted planes seem to have a lot of arcing lines, and you might need to work on your accuracy a little bit. Arc the other way, and take your time ghosting each line carefully."

Second version, after seeing the lines in the organic perspective exercise looking straight and accurate- "Your ghosted planes have a lot of arcing lines that aren't super accurate, but you improve on that later so good job!"

Being formulaic about it can make it a little bit tedious, but this way I can't miss any of the exercises in the lesson (and can reorder things chronologically in my critique even if their hosting site screws up the order)

In addition to what Yoyobuae has said, trying to find positive things to say about the person's work as well, we're not here to sugarcoat things but it's important to also point out what someone is doing well and should continue to do, because they may not even know they're doing that thing right. Also if you want to see critiques done well and the kinds of things we tend to pick about people's work, you can look over the critiques provided by myself, Svendogee, Sluggydragon, qzhans or even Uncomfortable himself.

Thanks Raynerain for asking this, and everyone who replied. I was wondering about this too and the info here is super helpful. I thought it might be better to wait until I'd passed some later lessons before offering critiques on lesson 1 but it seems not.