If you want to use the page you're working on as a template for future Notebooks, follow these steps and you'll be able to create the template from this existing page and use it again and again:
1. Export that single page in PDF Flattened format to the Files app. You can choose a cloud storage folder as a location for your template.
2. Add that template to Notebook Templates.
Download more paper templates and covers for GoodNotes here: How can I get more templates for GoodNotes?