Assuming you already have been granted access to the site for editing, just visit the root page of your site and add the string "/login" to the end. For example, if you were to login to the University Communications site, you would visit this URL:
Note that login is attached to the our central authentication service (CAS). So, if you are logged into PDX Gmail—or any other CAS-authenticated service—visiting the login page will just show the login page and the fact that you are logged in:
Note that the page is mostly blank and the administrative menus are across the top. This is the expected result.
If were not previously logged in, you will be redirected the the CAS login page, then returned to your site as an authenticated user. You will see the administrative menus across the top of the page.
If you encounter issues logging in, please drop a line to firstname.lastname@example.org.