wordpress-register-new-sidebar.php

//Register custom sidebar
function theme_slug_widgets_init() {
  register_sidebar( array (
    'name' => __( 'Custom Sidebar', 'theme_slug' ),
    'id' => 'custom_sidebar',
    'before_widget' => '<aside id="%1$s" class="widget %2$s" role="complementary">',
    'after_widget' => "</aside>",
    'before_title' => '<h4 class="widget-title">',
    'after_title' => '</h4>',
  ));
}
add_action( 'init', 'theme_slug_widgets_init' );