When accessing a discussion set as gradable, you can easily switch between the normal view and the grading view: Simply click on the Grades and Participation tab.

From there, you will see the list of students and can access their contributions to grade them.

Click on a student's name to view their contributions, read them, and grade them. In addition to the score, you can add feedback by clicking the speech bubble icon.

To navigate between students, use the arrows with their names at the top. When you finish, return to the list by clicking the X to close the window. You will see the students with their assigned grades. From there, you can choose whether or not to publish them.