Copy the module into <thelia_root>/local/modules/ directory and be sure that the name of the module is Paypal.
Activate it in your thelia administration panel

Composer

Add it in your main thelia composer.json file

composer require thelia/paypal-module:~2.0.0

II) How to use

To use the module, you first need to activate it in the back-office, tab Modules, and click on "Configure" on the line of paypal module. Enter your paypal login informations and save. Don't forget to do some fake orders in sandbox mode ( check "Active sandbox mode" box in tab Configure sandbox in the configuration page, and save ).

III) Integration

There is an integration example in the directory /templates/frontOffice/deufault You must do a page containing a internal error message called gotopaypalfail.html and a order cancel page called ordercanacled.html