Create a Custom Widget

<?php
class Custom_Message_Widget extends WP_Widget {

    function __construct() {
        parent::__construct(
            'custom_message_widget', // Base ID
            __('Custom Message Widget', 'text_domain'), // Name
            array('description' => __('A Custom Widget that displays a message.', 'text_domain'),) // Args
        );
    }

    public function widget($args, $instance) {
        $title = !empty($instance['title']) ? $instance['title'] : __('New title', 'text_domain');
        $message = !empty($instance['message']) ? $instance['message'] : __('Your custom message here', 'text_domain');

        echo $args['before_widget'];
        if (!empty($title)) {
            echo $args['before_title'] . $title . $args['after_title'];
        }
        echo '<p>' . $message . '</p>';
        echo $args['after_widget'];
    }

    public function form($instance) {
        $title = !empty($instance['title']) ? $instance['title'] : __('New title', 'text_domain');
        $message = !empty($instance['message']) ? $instance['message'] : __('Your custom message here', 'text_domain');
        ?>
        <p>
            <label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?></label>
            <input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo esc_attr($title); ?>">
        </p>
        <p>
            <label for="<?php echo $this->get_field_id('message'); ?>"><?php _e('Message:'); ?></label>
            <textarea class="widefat" id="<?php echo $this->get_field_id('message'); ?>" name="<?php echo $this->get_field_name('message'); ?>" rows="5"><?php echo esc_textarea($message); ?></textarea>
        </p>
        <?php
    }

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

function register_custom_message_widget() {
    register_widget('Custom_Message_Widget');
}
add_action('widgets_init', 'register_custom_message_widget');