Adding DomainKeys Identified Mail (DKIM) email verification to your Google Apps account
Introduction
The following is a pictorial guide to setting up DKIM for an already existing Google Apps account with your DNS hosted by any cPanel provider. The majority of the instructions and information provided here can also be found on the official Google support page, found at: http://www.google.com/support/a/bin/answer.py?answer=174124
Requirements
- You must already have a Google Apps account that is setup for and working with your email. This guide will not cover registration for Google Apps, or setting up any other aspect of Google Apps mail.
- You must be an administrator for the Google Apps account in question. Simply having an email account on Google Apps is not enough.
- You must be able to create a TXT DNS record for the domain(s) that you wish to have DKIM enabled for. I will cover doing this within cPanel, however it is not difficult to do at NameCheap, GoDaddy, or most other DNS control panels. Contact your DNS hosting provider if you need further assistance.
Before we begin
You will need to login, and verify that some settings are correct.
Click the "Manage this domain" link once logged into your Google Apps email.
Click on Domain Settings
“Enable pre-release features” must be checked. You must chose “Next generation” from the control panel options. In the future, these settings may not be required for DKIM support.
Let's enable DKIM
Click on the "Advanced Tools" menu.
Click on "Set up email authentication (DKIM)"
Chose the domain you want to enable DKIM for and Click the link to generate the DNS record.
Chose a prefix and click "Generate" - The default ("google" with no quotes) is fine, unless you know better.
We will be copy/pasting these into a new TXT DNS record.
Setting up the DNS record within cPanel
Start by logging into cPanel. You're on your own for this step 
Log into cPanel
Find the Advanced Zone Editor Icon and click on it
Setting up the DNS TXT Record. See comments in the image for full details.
Click Start Authentication back in the Google Apps control panel. It can take 1-2 days for it to take effect.
You're done!
That's all you need to do. It can take up to 24-48 hours for these changes you initiated to take effect for all of the internet, but it generally doesn't take that long. Give it a few days and you should be well on your way to helping fight off spammers.