You can access assignments using the "Assignments" (or equivalent) tab at the top of the course page and clicking on the assignment you wish to complete.
Once you have clicked on the assignment, select "get started" to open submission:
Once you have started the assignment, you may upload your documents from your computer by clicking 'Choose File'. Occasionally, if the assignment has the option to be made public, you can adjust your submission privacy level.
To submit your assignment, click the 'Save and Submit button'.
To confirm that your assignment has been submitted, look for a green check next to the assignment title: