By using custom domain name(domain.com) to point to Google Hosted Apps email server in cPanel, you may refer to the step as below and change the MX record record from your cPanel.

Caution: Please do not modify the MX record if you are not sure on how to change it.

1. Login to your cPanel account.

2. Click Mail.

3. Click MX Entry. Select a domain name that you wish to update.

4. For Email Routing, choose on “Remote Mail Exchanger” and click on Change.**Please tick on Remote Mail Exchanger as the email service is pointing to an external server**

5. Add the MX record with the Priority and Destination. Below is the priority and destination address of Google Apps Mail:

     1   (priority) aspmx.l.google.com

     5   (priority) alt1.aspmx.l.google.com

     5   (priority) alt2.aspmx.l.google.com

    10 (priority) alt3.aspmx.l.google.com

6. Click Add New Record.

7. Kindly allow few hours for the DNS propagation.