Select the payment gateway you want to integrate with. Popular options include PayPal, Stripe, Authorize.Net, and more.
Obtain the API credentials from your payment gateway provider. This usually includes a combination of a Merchant ID, API Key, and Secret Key.
Enter the obtained API credentials in the corresponding fields in the WHMCS payment gateway settings.
Most payment gateways provide a test/sandbox mode. Enable this mode and perform a test transaction to ensure the integration is working correctly.
WHMCS usually provides options to configure how errors are handled and what notifications are sent to clients and administrators.
Ensure that your integration complies with security standards and regulations. Follow best practices to safeguard sensitive information.