At first, you need to install and activate fluentCRM plugin. Then In WordPress admin, go to Smartpay > Integrations. You will find available list of extensions that can be integrated with WP Smartpay. Then Activate the fluentCRM integration.
As simple as this. Now fluentCRM integration is ready to use with WP Smartpay.
If you enable fluentCRM integration, After every successful payment transaction in WP Smartpay a contact will be created in fluentCRM.