Create composer.json file with requisite library for ex:
{
"require": {
"mpdf/mpdf": "^6.1"
}
}
Execute the below command where composer.json exists:
composer install
In case of Drupal :
Use the web root folder of drupal to include autoload for ex:
define('DRUPAL_ROOT', getcwd());
require_once DRUPAL_ROOT . '/vendor/autoload.php';
In case of other systems: Use the root folder variable or location to include the autoload.php