Many of the grading tools in Canvas assume you have a live Internet connection. However, it is possible to do some grading tasks offline - you can download, mark up, and re-upload papers, and you can also enter grades into a file and upload them to Canvas later.
Downloading submissions, adding comments, and returning them to students
You can bulk download submissions when you have an Internet connection, make comments within the document, and re-upload the submissions to return the commented files to the students. If you do this, make sure you don't change the original names or formats of the submitted files. Doing so will break the association between the submissions and students. Also be aware that it can take several minutes to re-upload files - please be patient, and don't upload them more than once.
Keeping an external gradesheet
You may also want to record grades while offline. To do this, you can download a grading spreadsheet, make changes to it while offline, and upload the grades later. You will have opportunity to review changes made to the Gradebook when you upload the spreadsheet. Assignments with no grade changes will be hidden, so if you see an assignment column you have not made changes to, you will want to review this screen carefully prior to uploading the file.