Create the gateway extension. See Invoice payment extension details for information specific to this extension.

Upload the extension to the administration interface.

Enable the gateway for the sites that require it.

Add an invoice payment option to the checkout page.

Configure the Generic Payment webhook by specifying the gateway URL and the username and password. (This step is optional. See Validate the purchase order number.) Note that webhook settings are not site-specific. The configuration you supply applies to all sites that use this webhook.

Add invoice payment information to order pages.