Custom Post Templates provides a drop-down to select different templates for posts from the post edit screen. The templates are defined similarly to page templates, and will replace single.php for the specified post. This plugin will NOT switch the templates for the different posts in a listing page, e.g. a date or category archive, it will only affect the template used for single posts (i.e. you can choose a template which is not single.php).
Post templates, as far as this plugin is concerned, are configured similarly to page templates in that they have a particular style of PHP comment at the top of them. Each post template must contain the following, or similar, at the top:
Template Name Posts: Full Width
First note: Page templates use “Template Name:”, whereas post templates use “Template Name Posts:”.
Second note: You must have the custom post template files in your theme in the same directory/folder as your index.php template file, not in a sub-directory/sub-folder.