To create a template, first goto the template page (More information here)
Creating a new template
Click the New Template button in the top right hand corner.
Editing Template
Click the kebab menu next to a template, and click Edit template
Add default Category to Template
While you are editing a template, you can click the category (in the top left) and choose a default category for the template. All future pages created using that template will default to that category.
Saving Template
Once you are finished editing and categorizing the template, click Publish template!
Creating a template from an existing page
To create a template from an existing page, go to any page, click on the kebab menu in the upper right, then click Create template