How to use a PHP package via Github manually instead of composer

I have a very quick and simple question, I want to use this PHP package: https://github.com/zytzagoo/smtp-validate-email to validate if an email exist, but in the documentation of installation it only show how to install via composer and I want to install it manually (downloading the package and add it to my project).

In the example with composer it's like this:

composer require zytzagoo/smtp-validate-email --update-no-dev

And to use it is like this

require 'vendor/autoload.php';

use SMTPValidateEmail\Validator as SMTP_Validate_Email;

I just want to know what should I do for a manually installation instead of the composer