Step 1: Create a file new-template.php in theme folder
Navigate to wp-content->themes->twentytwenty( Note: Select any of the active theme in your wordpress), and create a new file new-template.php
Content of new-template.php is:
<?php /* * Template Name: New Template * Template Post Type: post */ get_header(); ?>
“Template Name” is important to set, this is the name which will show in the list of template available.
You can copy code of singular.php in new-template.php and customize it because both have same functionality i.e single blog post-presentation
Step 2: Test
Navigate to new post->Post Attribute->Template
Now your template “New Template” will be listed in template list.