Once you open a class, you are brought to the "Assignments" sub-tab by default.
From there select the "Create Assignment" button in the upper right.
Select your preferred assignment type.
Fill in the necessary fields: Assignment Name, Instructions, any Attachments, etc.
Hit "Next" to choose the date you want to assign it, as well as the time and date the assignment is due by.
Select your desired Grading Period and whether you would like to accept late submissions or not.
Hit "Create Assignment" to complete assigning it to the class.