You can attach a rubric to an assignment using the normal assignment creation workflow.
Use the + icon on the left sidebar and choose tab New Assignment. To attach a rubric to the new assignment, click on the Attach Rubric link located in the new assignment window.
From the Attach a Rubric modal, you can select a rubric from your collection of rubrics, or create a new one for your new assignment.
Attaching a Rubric after the assignment has been created
You can also attach the rubric after you've created the assignment. Click on the assignment you would like to add the rubric, and choose "Attach Rubric".
Now that you've attached the rubric to your assignment, your students can see what's expected of them. But, now you'll have access to that rubric when you go to grade the assignment!