To create a template, head over to your templates screen and click the "Create template" button.
Depending on which editor you've configured as the template editor, you'll either see an HTML editor or Markdown editor.
You can add "slots" to your template by entering the slot name between triple brackets. In the screenshot below, we've modified the Default template, and added a footer placeholder.
When you edit the content of the campaign that uses this template, you'll see a footer field.