From the main window, choose New, Use Template from the Compose menu, then choose the template you would like to use from the submenu.
The Compose window opens with the template loaded.
If the Compose window is already open, you can load the template by choosing its name from the Use Template submenu of the Format menu in the Compose window.
Edit the headers and body, as necessary, and send the message as usual.
For more details, See "Editing and Formatting Outgoing Messages".