Begin by clicking on the "Grades" tab in the blue Navbar. Then click on the settings cog at the top right of the page.

Next, click on the "Calculation Options" tab

Scroll down to the bottom of the page. Click on the radio button next to "Drop ungraded items" or "treat ungraded items as 0." If you want to tie "treat ungraded items as 0" to a due date, check the box next to "Automatically apply a grade of zero (0) to missing submissions once an assessment becomes overdue."