Further down you can grade the assignment using the drop down menu (if you have chosen a custom grading scale) or if you have chosen a number grade then you will get a small box to type the grade into.
There is also a box for putting your feedback into.
If you enabled the ability to add feedback files, you will see an upload box underneath the comments section for you to upload the feedback file to the student. This can be their own assignment, sent back to them with comments all over it.
Once you are done, either click "Save changes" if this is the only student you are giving feedback to, or "Save and show next" if you are grading all of your students.
Clicking "Cancel" or "Next" will lose your grade and feedback information!