You would want to apply the Universal Email server behavior to a page that will be accessed when you want to send out the emails. For example, if you would like to login to your admin area and send the emails out from there you can create a new newsletter send page and apply the Universal Email server behavior to it. You could add in a form to the page and set the trigger to the submit button being pressed.
On this page you would need to include a recordset that selects all of the users from your users table who are active subscribers. Once you have the recordset crafted you can reference it within the Universal Email server behavior to have it send to each entry.
If you are not too familiar with using UE 4 then you should take a look at the getting started guide for it here:
universal-email_gsg.php
This and other information is available on the Universal Email support page.
