create a child theme https://codex.www.ads-software.com/Child_Themes first and make the edits in there;
edit single.php in the child theme (copy it from the Twenty Eleven parent theme and save it in the child theme, using ftp or cpanel)
and add <?php get_sidebar(); ?>
before the line with ‘get_footer’;
create a new functions.php (using ftp or cpanel) in the child theme, and add <?php
in the first line.
add:
add_filter('body_class', 'adjust_te_body_class', 20, 1);
function adjust_te__body_class($wp_classes) {
if( is_single() ) :
foreach($wp_classes as $key => $value) {
if ($value == 'singular') unset($wp_classes[$key]);
}
endif;
return $wp_classes;
}
possibly for fine tuning, add a few styles to style.css of your child theme;
examples see in my article