You will just log in, and go to the screen where you added spelling levels to your student(s). From there, you can add grammar levels to your student accounts. When they log in after that, they will see the link to start the grammar lesson.
To get started, your student(s) will need to log in using the account(s) that you created for them. If you aren't sure of their username and/or password, they can be checked and/or reset from your instructor account.