Depending on the assignment, you can turn in a personal doc that your teacher assigned to you, create your own Google Doc, or add files to the assignment.
Go to classroom.google.com.
Click the class > Classwork > the assignment.
To attach an item:
Under Your work, click "Add or create" . Options you can choose include
Finding a file in your Google Drive
Entering a URL for a link
Selecting a File to attach
Creating a new Docs, Slides or Sheets
Click Turn In and confirm.
The status of the assignment changes to Turned in.