Since there are many different ways to use the Assignment activity, there are many different ways to grade submissions. One quick-start method is described here. Look for more options (including rubrics) on the Advanced Grading - Assignments page
To grade an Assignment , go to your course and:
Click the link to open the Assignment
Click the View all submissions link
This opens a grading page for the assignment. You can select which block or period to grade - or see all your students at once.
Download any file submissions individually from this page, or download all submissions at once (toward bottom of page).
For online text assignments, the students response is displayed on this page under File submissions instead of the file to download.
Turn on Quick Grade (near bottom of page) to enter grades directly on this page.
Turn on Quick Grading (near bottom of page).
Be sure to save your grades.
And you can elect to have students notified by email that their work has been graded.
Download any file submissions individually or download all submissions at once (toward bottom of page).