This guide will walk you through setting up a SPF record for your Google Apps mail.
In order to create a SPF record for your domain, you would have to log in to cPanel, then navigate to Advanced DNS Zone Editor. If you have more than one domain on your cPanel account, choose the appropriate one from the dropdown menu.
Fill out the form with the following info:
Name - yourdomain.com*
*Where 'yourdomain.com' is replaced by the actual URL of your site.
TTL - 3600
Type - TXT
Address - v=spf1 include:_spf.google.com ~all