WordPress.org

1. Upload the folder 'related-posts' with all files to '/wp-content/plugins' on your webserver
2. Activate the plugin through the 'Plugins' menu in WordPress
3. Make sure that your posts have tags (no tags = no relations).
4. Add the widget "related posts" to your sidebar and configure it as you like, use the placeholder "%RELATEDPOSTS%" within your posts, or add the following code to the single.php of your theme: Related Posts:' ); ?>

German screenshots of the installation, configuration and usage of this plugin: related-postsinstallde.jpg