To upload student feedback to each student's assignment submission follow these steps:

1) Click into the Assignment Area
2) Click on View / Grade All Submissions
3) When the student list loads, under the grade column there is an edit icon which looks like a pencil
4) Clicking on this icon will bring up the page for that student where you can submit the feedback file. This is uploaded by dragging the feedback file into the Feedback Files box within the Grades Heading. Alternatively, instead of dragging, click the icons with the plus symbols located on the Feedback Files menubar to launch the FilePicker where you can browse and select the file.