Now these template files will be totally empty. As a starting point, you can copy the contents of your single.php file from your theme folder and paste them inside each of these templates.
You can now edit these templates to make your desired changes.
Once you are done, you can go to your website and view a post. It will use the template that you have created for the category where this post is filed.
Now let’s suppose you have a post filed in two categories News and Travel Tips. WordPress will automatically show the template for ‘News’ because it appears first in alphabetical order.