Step::1 Log into your WHMCS.
Step::2 Hover over the Setup and Payments options and click on Payment Gateways.
Step::3 Go to All Payment Gateways tab.
Step::4 Select PayPal payment gateway.
Step::5 A configuration page will appear.
Step::6 Click on Save to set up the gateway.
That's It !Check out the video tutorial to get the procedure in detail. Read the blog as well.