To create your own custom mass email, please first log in to your portal
Then navigate to Communication > Newsletters
Then click on the New Newsletter button at the top
From there, you would edit the Subject of the Email and add everything you want in that email into the Intro Text section. You would not need to add any articles if you are treating this as a regular email.
Once you are done adding the content into the email, scroll down and click on Schedule or Send Now button.