In the top tool bar, find "Write." This will drop down with various posting options. To assign work for your students, choose "Create exercise."
Here you can give a title to your assignment, and fill in the necessary information for the completing it.
At right, you will see "classroom" and "tags."
Select the classroom the assignment is to be posted to.
Next, give it a new tag-- Select "other" in tags, and type in your new tag. This is how the students' work will be organized so you can find responses to the proper assignment.
Make sure to mention in the instructions which tag to select when responding to the assignment.
And finally, Save.