Lead Developer
I’m not sure if there’s one specific guide, but there’s tons of information on all of that separately if you Google it.
Create a page template simple means copying an existing template in GP (page.php) for example, and uploading it to your child theme, then changing the file name and adding the page template header: https://developer.wordpress.org/themes/template-files-section/page-template-files/page-templates/
What are you doing that requires a custom page template?