Below is one way of giving feedback described. Another option is Using SpeedGrader.

How to download students submissions was is described in the article Finding student submissions. Once you have downloaded and unzipped all the submissions you can open them up in your word processor (make sure students submit files in a format you can work with, e.g. if you are using Word make sure they submit doc. or .docx files). Mark, edit, comment, give feedback using the word processor tools you prefer and don't forget to save.

Note!

Do not change the file names (if you let them remain the same only a single upload is necessary in the coming steps).

When you are done giving feedback you zip the files back together by right clicking your folder with submissions, choose 7-zip and then Add to "folder-name.zip", where folder-name will be replaced with name of the folder on your computer.

Add submissions to a zip file.

A zip file will be created. Head back to into Canvas and visit the assignment details page and click the new option in the right side menu - Re-upload submissions (the same option is available from the Gradebook).

Re-upload submissions with feedback.

Choose your recently created zip-file, click Upload files and the your feedback will automatically be distributed to the right student (if you haven't fiddled with the file names).

Note!

With this method students receive feedback as a comment with an attached file to their submission. They have NOT been graded since no result has been supplied.

After giving feedback with this method you can grade your students by either Using SpeedGrader or from The Gradebook.