Watch the following video for a tutorial.
Click on the assignment in Google Classroom.
Click "+ Add or create".
Click "Google Drive".
Find and double-click your completed homework.
Your Google Doc is now attached the assignment and ready to be submitted.
Click on the assignment in Google Classroom.
Click "+ Add or create".
Click "Doc".
Then click on the newly created document.
Type your homework assignment as the teacher directed.
Click "Turn In".
Double check the assignment. If it says Missing, remember to click "Turn In".