Attaching an assignment
You can attach one or more files to your work. You can also open new files in Google Docs, Slides, Sheets, and Drawings, work in them, and then attach them to your assignment. However, you can’t attach a file that you don’t own.
Go to classroom.google.com.
Click the class > Classwork.
Click the assignment > View assignment.
To attach an item:
Under Your work, click Add or create > select Google Drive , Link , or File
Select the attachment or enter the URL for a link and click Add.
6. Click Turn In and confirm.
The status of the assignment changes to Turned in.
If your teacher attached a document with your name in the title, it’s your personal copy to review and edit. As you work, your teacher can review your progress before you click Turn in.
Go to classroom.google.com.
Click the class > Classwork.
Click the assignment > View assignment.
Click the image with your name to open the assigned file.
Enter your work.
On the document or in Classroom, click Turn in and confirm.
The status of the assignment changes to Turned in.
Go to classroom.google.com.
Click the class > Classwork.
Click the assignment > View assignment.
Click the form and answer the questions.
Click Submit.
If the form is the only work for the assignment, the status of the assignment changes to Turned in.
If there's more work to do for the assignment, click Open assignment.
Source: https://support.google.com/edu/classroom/answer/6020285?co=GENIE.Platform%3DDesktop&hl=en&oco=1