Login to your WHMCS admin control panel and navigate to Configuration > System Settings > Automation Settings (Setup > Automation Settings in WHMCS 7.10 and earlier).

Here the cron command is provided to you ready to simply copy and paste into your web hosting control panel. Copy this to your clipboard now.

Log in to cPanel and navigate to Advanced > Cron Jobs.

Under the Add New Cron Job option, select "Once per Five Minutes"

Paste in the cron command you copied in step (1). Then click Add New Cron Job.

You will see a success notification as follows to confirm the cron has been setup successfully.

That's it. Your cron is now configured.