The first step is to have a rubric created, after that you will need to assign the rubric to the item.
At the bottom of your assignment click on the + Rubric button:
In the upper right of the Rubric window click on Find a Rubric.
In the popup window locate your course and then the rubric you want to use. Select it and then click Use This Rubric.
Now that your rubric is attached, in the upper right corner of the rubric click on the pencil icon to edit the settings.
At the bottom of these settings make sure to check the box next to Use this rubric for assignment grading then click Update Rubric.
Now when you put the grades in the student’s rubric it will show up in the grades area automatically.