This is a merchant gateway for Blesta that integrates with Cornerstone.
You can install the gateway via composer:
composer require blesta/cornerstone
Upload the source code to a /components/gateways/merchant/cornerstone/ directory within your Blesta installation path.
Log in to your admin Blesta account and navigate to
Settings > Payment Gateways
Find the Cornerstone and click the "Install" button to install it