Take the file the custom post type is registered in, put it in the plugins folder, and add a comment at the top, e.g.:
<?php
/**
* Plugin Name: David A. Frenchs Custom Post Type Plugin
**/
Save and visit the plugins page to activate it, and that’s it! A plugin is just a file with a comment at the top. You can put it inside a folder and include other files into your plugin too, WP will search for the comment at the top of the file and load that one.
Just remember to prefix your plugins name, if somebody else has the same idea and uploads a plugin with the same folder/file name to wordpress.org it’ll trigger the updater