WordPress twenty eleven sidebar on Pages

Bainternet's tutorial is very good.

Another tutorial, which worked for me (and is actually a little simpler) is here:

http://futurewebblog.com/add-sidebar-support-posts-twenty-eleven-theme/

And here's what I added to my functions.php file, after studying both tutorials:

    /*
     * Fix for no sidebar in single posts
     * From http://futurewebblog.com/add-sidebar-support-posts-twenty-eleven-theme/
     */

    /* In child themes the functions.php is applied before the parent
     * theme's functions.php. So we need to wait for the parent theme to add 
     * it's filter before we can remove it.
     */
    add_action( 'after_setup_theme', 'my_child_theme_setup' );

    /**
     * Removes the filter that adds the "singular" class to the body element
     * which centers the content and does not allow for a sidebar
     * By http://futurewebblog.com/add-sidebar-support-posts-twenty-eleven-theme/
     */
    function my_child_theme_setup() {
        remove_filter( 'body_class', 'twentyeleven_body_classes' );
    }

I didn't have to make any changes to CSS, and it worked!

HTH. Jeff

Funny you should ask, I've just posted an How to tutorial on how to add a sidebar on posts and pages in Twenty Eleven