In your theme functions.php create a function
function cosmowhiz_setup()
inside the function add the following code to register menus as follow:
if ( ! function_exists( 'cosmowhiz_setup' ) ) : function cosmowhiz_setup() { register_sidebar( array( 'name' => __( 'Sidebar', 'cosmowhiz' ), 'id' => 'sidebar-1', 'description' => __( 'Add widgets here to appear in your sidebar.', 'twentysixteen' ), 'before_widget' => '<section id="%1$s" class="widget %2$s">', 'after_widget' => '</section>', 'before_title' => '<h2 class="widget-title">', 'after_title' => '</h2>', ) ); register_sidebar( array( 'name' => __( 'Content Bottom 1', 'twentysixteen' ), 'id' => 'sidebar-2', 'description' => __( 'Appears at the bottom of the content on posts and pages.', 'twentysixteen' ), 'before_widget' => '<section id="%1$s" class="widget %2$s">', 'after_widget' => '</section>', 'before_title' => '<h2 class="widget-title">', 'after_title' => '</h2>', ) ); register_sidebar( array( 'name' => __( 'Content Bottom 2', 'twentysixteen' ), 'id' => 'sidebar-3', 'description' => __( 'Appears at the bottom of the content on posts and pages.', 'twentysixteen' ), 'before_widget' => '<section id="%1$s" class="widget %2$s">', 'after_widget' => '</section>', 'before_title' => '<h2 class="widget-title">', 'after_title' => '</h2>', ) ); } endif; // cosmowhiz_setup