To grade a submission, click on the "Grades" tab in the blue navbar in your course shell. 

Find a submission you'd like to grade and click on it. Unread submissions have an orange dot next to them.

Click on the file at the top left of the page if it's a file submission. If it's a text submission it will appear automatically.

Use the tools at the top of the document to draw, post notes, and create text boxes on the submission itself.

On the right side of the page you can give the submission an overall grade, written feedback, upload files, or quick link to course content. You can also record an audio or video message. When you are finished click the blue "Publish" button on the bottom right of the page or "Save Draft."