Adding a number next to each comment

I’ve got a blog that has a high amount of comments and I want to add a number next to each comment posted.
Like it does on here:https://wphacks.com/how-to-adding-comment-numbers-to-your-wordpress-theme/
(That is for an old version of WP)

The Sliding Door theme uses the same functions and comments.php that WordPress Twenty Eleven has and I don’t know where to start.
Could anybody help ?