​Here are the steps:
1. Go to
http://www.google.com/a/cpanel/domain/new
2. Fill in your domain and details required
3. Change your MX entries in your domain into the following:
aspmx.l.google.com - priority 10
alt1.aspmx.l.google.com. - priority 20
alt2.aspmx.l.google.com. - priority 30
aspmx2.googlemail.com. - priority 40
aspmx3.googlemail.com. - priority 50
4. To simplify your access to google apps, go to Service Settings -> Email and click on Change URL and select Custom and follow the instruction.
5. Set the CNAME from your mail.yourdomainname.com to google CNAME, i.e ghs.google.com