To add comments to a student's paper:
Comments can be up to 1000 characters in length, which translates to roughly 10-12 lines of text in a word document.

Click on the grade icon  to access a submission:
grade_icon

You can enter a comment at any point in the submission. Click on a specific section of the submission and then enter your text:
enter_comment


You can then select save, or you can opt to turn the comment into a reusable quickmark by selecting More Options, entering a title, and then selecting
a  set to store the comment
create_quickmark

Once created, a quickmark can be added to any submission; it can also be edited so that it can be repurposed. To add a quickmark comment, simply right-click it from the quickmark sidebar and drag it to the appropriate point in the paper.

You can also enter a text comment on a submission page. Select The text icon in the quickmark menu and click in the paper to add your comment:

text_comment
Delete a comment by hovering in it then selecting the trash can icon to
the top right:
cc delete_comment