By Writing Code in the Theme’s functions.php File

// Define the widget class
class My_Custom_Widget extends WP_Widget {
    // Widget settings and initialization
    function __construct() {
        parent::__construct(
            'my_custom_widget', 
            esc_html__('My Custom Widget', 'text_domain'), 
            array('description' => esc_html__('A Custom Widget',
                                              'text_domain'), )
        );
    }

    // Front-end display of widget
    public function widget($args, $instance) {
        echo $args['before_widget'];
        if (!empty($instance['title'])) {
            echo $args['before_title'] . apply_filters('widget_title',
                 $instance['title']) . $args['after_title'];
        }
        echo esc_html__('Hello, World!', 'text_domain');
        echo $args['after_widget'];
    }

    // Back-end widget form
    public function form($instance) {
        $title = !empty($instance['title']) ? $instance['title'] :
                  esc_html__('New title', 'text_domain');
        ?>
        <p>
        <label for="<?php echo esc_attr($this->get_field_id('title')); ?>">
        <?php esc_attr_e('Title:', 'text_domain'); ?></label> 
        <input class="widefat" id="<?php echo esc_attr($this->get_field_id('title')); ?>" 
                               name="<?php echo esc_attr($this->get_field_name('title')); ?>" 
                               type="text" value="<?php echo esc_attr($title); ?>">
        </p>
        <?php
    }

    // Save widget form values
    public function update($new_instance, $old_instance) {
        $instance = array();
        $instance['title'] = (!empty($new_instance['title'])) ?
                             strip_tags($new_instance['title']) : '';
        return $instance;
    }
}

// Register the widget
function register_my_custom_widget() {
    register_widget('My_Custom_Widget');
}
add_action('widgets_init', 'register_my_custom_widget');