A zip file will be created that you can save to your local computer. You can then extract the contents of the zip file. Inside there will be a main folder with the assignment name; inside that will be the grade file (if that option was selected) and a folder for each student. The contents of the student folder will depend on the options selected. There may be subfolders for Submission and Feedback Attachments; if you wish to provide comments, edit the comments.txt file. Grades should be entered in the grades.csv file.
Once you have completed grading, you need to zip the files. Make sure you zip the main folder with the assignment name.
Use this guide for help uploading your files to OneDrive.