Begin by clicking on "Course Admin" and scrolling down to "Discussions." From there, click on the caret next to the discussion topic you want to grade and click on "assess topic"

Scroll down and click on the student you'd like to grade.

Enter your grade/feedback, then click on the blue "Publish" button if you're ready or "Save Draft" if you want to publish it later.

When you're ready to publish feedback, in the "assess topic" screen, select all students to publish grades/feedback to and click on the blue "Publish" button.