As a teacher, how do I mark students' Google Classroom assignments using Lumin?

Firstly, ensure you have the Lumin app installed as a Google Drive extension. Find out how here.
Go to Google Classroom and select the class/project where you issued the assignment.
Select the 'People' tab.
Click on the Student's name.
Click on the turned-in document.
Select the 'Open with' dropdown, and choose Lumin.
Connect Lumin to Google Drive.
Mark your students' work using the Lumin annotation tools (free text, comment, draw, and shapes).
Click Auto Sync to ensure your work is saved back to Google Drive.
Go back to Google Classroom, refresh the page, and select 'Return' to hand back your annotated assignment to your student.
Go back to your Classroom and repeat the process for any other turned-in assignments.

Lumin integrates with Google Classroom, allowing you to mark your students' turned-in assignments, and then sync the marked copy back to Google Classroom for grade dispersal.