Create a free Fondy account https://portal.fondy.eu/mportal/
Copy the module folder into the <thelia_root>/local/modules/ directory and make sure that the name of the folder is Fondy or create zip archive which contains folder Fondy/{all files from this repository} and install from administration side.
Activate Fondy in your Thelia administration panel.

Composer

Add it in your main Thelia composer.json file

composer config repositories.cloudipsp git https://github.com/cloudipsp/thelia.git
composer require cloudipsp/thelia

How to use

To use this module, your first need to activate if in the Back-office, tab Modules, then click on "Configure" on the Fondy module line. Enter your Merchant_id, secret_key and save.