Click and drag one of your rubric elements from the right-hand list. The rubric elements are the blue buttons that look like this:
Drag the rubric element button over the essay.
Sentence-level rubric elements will highlight the text sentence-by-sentence as you drag the button over the text. Paragraph-level rubric elements have a "[ ¶ ]" symbol and will bracket each paragraph as you drag over the text.

Release the rubric element button when the desired text is highlighted. The evaluation options for that rubric element will then pop up:

Now evaluate the quality of the selected rubric element. Choose a feedback comment from the appropriate quality level column or add a new comment as needed.