How do I add a Content Template in the Rich-Text Editor?
In addition to selecting a Site Template when you create a new site, you can add a Content Template in the Rich-Text Editor.
Step-by-Step Instructions
Select the Templates icon.

The Templates icon looks like a piece of paper with the corner folded down.
Select the Content Template.

Select the desired template. The Site Templates you can select when you create a new site are available among the template options.
Important: By default, the Replace actual contents box is selected. If you have content in the editor that you do not want to delete, uncheck this box.