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 a drag-and-drop (Unlayer) editor.
You can add "slots" to your template by entering the slot name between triple brackets, for example:
Once you create an email with this template, you'll get a field with your configured content editor for each slot.
You can also create templates without any slots, these will copy the full HTML contents to the email when used.