Creating Child Theme for Custom Theme

Hi, I’m working on a site that a developer converted to WordPress by modifying the Twenty Eleven directly. The version of WordPress is 3.3.2, and I’m trying to update the version so that I can use certain plugins, but I can’t get the update to work without the whole site blowing up.

So I tried creating a child theme of the customized Twenty Eleven theme following the instructions on creating child themes from the Codex as best as I could. I created a new folder under Themes with the name twentyeleven-child, I copied the customized style.css, pasted in the new directory, and modified the theme name to ‘Twenty Eleven Child’ and the template to ‘twentyeleven’. I then copied all of the other files from the customized parent twentyeleven folder to my twentyeleven-child folder, except for functions.php. I activated my child theme and it worked fine (when functions.php was copied to the child folder, it didn’t work). I then downloaded the most recent version of twentyeleven, and copied the downloaded files into the parent twentyeleven folder to update the version. Now the layout of the site is all messed up.

What do I need to do to create a child theme that is a copy of the custom parent theme, and be able to update WordPress? Is there a certain subset of files that I need to copy over to the child theme directory?

Thanks for you help.