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)