This article shows how to add custom templates in GoodNotes 4. For the GoodNotes 5 instructions, please view this article.
Any single-page PDF files can be used as a template. To add a custom template from iTunes or other cloud services, please open the Template Library from Options > Template Library, select a folder, tap Edit, and then tap +.
The easiest way to create a custom template is to create an empty page with one of the built-in templates, and then export to PDF. Then use Adobe Illustrator or another vector application to edit the PDF file.