To create a custom page template make a new file starting with a Template Name inside a PHP comment. Here’s the syntax:
<?php /* Template Name: My Custom Page */
Once you upload the file to your Theme’s folder, the template name, “My Custom Page”, will list in the Edit Page screen’s Template dropdown. (The select list has a maximum width of 250px, so longer names may be cut off.)
A custom page template file can be in a sub-folder, or, if using a Child Theme, in its Parent Theme’s folder.