1. Download latest Plisio Gateway osCommerce plugin release.
2. Extract downloaded zip. Upload includes directory and plisio_callback.php file to the root directory of your osCommerce installation.
3. Login to your osCommerce admin panel and go to Modules » Payment. Click Install module, then click Plisio Payment Gateway and click Install module again.
4. Go to Modules » Payment, click Plisio Payment Gateway and click Edit. Set Enable Plisio module to True. Enter your API key and configure other extension settings. Click Save.