Create widget by write code in functions.php of specified theme.

functions.php

/*
 * example of simple widget
 */

// Creating the widget popularNews 
class nameOfWidget extends WP_Widget
{

    function __construct()
    {
        parent::__construct(
// Base ID of your widget
                'nameOfWidget',
// Widget name will appear in UI
                __('Название виджета', 'nameOfWidget_domain'),
// Widget description
                array('description' => __('Описание виджета', 'nameOfWidget_domain'),)
        );
    }

// Creating widget front-end
// This is where the action happens
    function widget($args, $instance)
    {
        $title = apply_filters('widget_title', $instance['title']);
// before and after widget arguments are defined by themes
        echo $args['before_widget'];
        if (!empty($title))
            echo $args['before_title'] . $title . $args['after_title'];

// This is where you run the code and display the output
        ?>


        /*
        
        There you need to paste output code
        something like:

        <div>
            <b>Out this string like widget</b>
        </div>

        */



        <?php
        echo $args['after_widget'];
    }

    // Widget Backend 
    function form($instance)
    {
        if (isset($instance['title']))
        {
            $title = $instance['title'];
        }
        else
        {
            $title = __('New title', 'nameOfWidget_domain');
        }
        // Widget admin form
        ?>
        <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>
        <?php
    }

// Updating widget replacing old instances with new
    function update($new_instance, $old_instance)
    {
        $instance = array();
        $instance['title'] = (!empty($new_instance['title']) ) ? strip_tags($new_instance['title']) : '';
        return $instance;
    }

}

// Class wpb_widget ends here
// Register and load the widget
function nameOfWidget_load_widget()
{
    register_widget('nameOfWidget');
}

add_action('widgets_init', 'nameOfWidget_load_widget');