Connect to your server via (s)FTP, scp or alternative file transfer method.
Navigate to the location of your site; common locations are:
Copy the files to the base of the directory (they'll place themselves in the correct directories for you).
Navigate to www.yoursite.com/admin, and login to the Admin interface.
Hover over the 'Extensions' menu, this will expand; click on the 'Modules' menu item.
You should see 'Expressly' in the available modules table - click 'Install'.
We're not done yet! You still need to add your API Key. You can get it by signing-in or registering on http://portal.buyexpressly.com/. After installing, you'll be redirected to the module configuration page where you can add your API key.
If all went well, you should be installed, registered, and ready to go!