A few things. First is to check out the exercise pages and the common mistakes section. Most mistakes are written there, and they are done a lot on the submissions, so only with using those you can provide good feedback.

And second one is to check other people's critique, the teaching asistants if you can, as their work is reviewed by uncomfortable, you can trust they give quality reviews. Some of the TAs are svendogee, meta, qzhans, nihlex or sluggydragon.

Here's meta's page for example, you can check her reviews there: https://drawabox.com/community/sketchbook/meta

There are common mistakes outlined in the lessons that students usually run into. There's always the nuance of how exactly the mistake is made and looking for patterns and pointing them out could be of help to the person you're giving critique to. Perhaps they tend to have the further edges of the box wider than the closer ones, or perhaps the conversion of the furtherest edge is not being aligned with te closest one. If there's a pattern, there's a good chance that the student does not see the issue.

One more thing that is good to emphasize is progress over result - we want people to follow the outline of the exercise as it will build their skill over time. So if they might sound discouraged in the submission, remind them that progress is being made.