Connect to your server via (s)FTP, scp or alternative file transfer method.
Navigate to the location of your site; common locations are:
If you osCommerce installation is installed under catalog then copy the files to the base of the directory (they'll place themselves in the correct directories for you providing your).
Alternatively if you have moved the catalog files into the root of the site you should instead copy the content of the catalog directory in the distribution to the directory
Navigate to www.yoursite.com/admin.php, and login to the Admin interface.
On the left-hand menu, you should see a new block named 'Expressly'. Navigate to the only Expressly menu item: Preferences.
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.
On the left-hand menu go to Modules > Header Tags. On this page click on Install Module. Select Expressly Migration Trigger and click on Install.
If all went well, you should be installed, registered, and ready to go!