Create the random subdomain (under domains) that google gives you.
Then under DNS overides choose the new subdomain and complete the CNAME record to be google.com
Check that this has taken affect. when you press the I have done this button google then checks. If you hae not done this it will assume that you do not own the domain and not allow you to continue.
For easy access to your mailbox, create a new subdomain (under domains) mail.yourdomain.com. Then under DNS overides set the CNAME to ghs.google.com
Then under domains for yourdomain.com create these MX records