Some email programs allow the user to create a "signature" that is appended to the bottom of every outgoing email. In Direct Mail, we take this idea one step further. Instead of creating just a signature, Direct Mail allows you to create an entire message, and then convert that message to a template that you can reuse over and over again.
To create a template, first create a new message in Direct Mail. You can use other pre-existing templates as a starting point for your message, if you like. Once you've designed your message the way you want it, choose Message > Create Template from Message from the menu bar at the top of the screen and then give your template a name.
After you've created your template, it will appear in the template chooser alongside the rest of Direct Mail's built-in templates. You can even make that template the default for all new messages in the Preferences window.