If I understand you correctly.
Just create a php file in your theme directly called “pagename-template.php”
Replacing “pagename” with what you want it to be like “about” or “blank” etc
In your PHP file – simply put the following at the top of your file in comments:
/*
*
* Template Name: Blank
*
*/
Include your header and footer functions and The Loop if you want to use the WSIWIG editor on the “Edit Page” page if need be.
Then go create a new page in the WordPress Dashboard and you’ll notice a template section under the Page Attributes box on the left with the title “Templates” and a dropdown, you’ll see “Blank” as a selectable item there from now on.
Happy coding!