Add a Custom Widget Area

<?php
function custom_widget_area() {
    register_sidebar(array(
        'name'          => __('Custom Widget Area', 'textdomain'),
        'id'            => 'custom-widget-area',
        'before_widget' => '<div class="custom-widget">',
        'after_widget'  => '</div>',
        'before_title'  => '<h2 class="widget-title">',
        'after_title'   => '</h2>',
    ));
}
add_action('widgets_init', 'custom_widget_area');