First, go to my.bgsu.edu and log in:

In my case, the my.bgsu.edu page looks like this:

Above the words Welcome, Craig! there is an orange tab called My Web Courses. Click on it to get a page similar to this:

I am teaching several courses this semester. If I click on Curric Anal Clasrm Behavr, I get:

I have not posted any announcements for this class. If I click on the Communications button on the left, I get:

(If I clicked Roster here, then click the List All tab, then again List All, I would get a roster with email addresses but no P00 numbers).
Now I click Send E-mail and get:

Finally, I click All Users to get:

Now I can type the message and add attachments if I like.