1. Open the Settings application on your device’s home screen.
2. Open Mail, Contacts, Calendars.
3. Tap Add Account….
4. Select Microsoft Exchange. OS 4.0+ now allows multiple Exchange accounts. However, if you’re on a device that doesn’t let you add a second account, you could also use CalDAV to sync Google Calendar and IMAP to sync Gmail.
5. In the Email field, enter your full email address.
6. Leave the Domain field blank.
7. Enter your full Google Account email address as the Username.
8. Enter your Google Account password as the Password. (If you are a 2-step verification user, please use an application-specific password instead.)
9. Tap Next at the top of your screen.
9a. Choose Cancel if the Unable to Verify Certificate dialog appears.
10. When the new Server field appears, enter m.google.com.
11. Press Next at the top of your screen again.
12. Select the Google services (Mail, Calendar, and Contacts) you want to sync.
13. Unless you want to delete all the existing Contacts and Calendars on your phone, select the Keep on my iPhone option when prompted. This will also allow you to keep syncing with your computer via iTunes.
If you want to sync only the My Contacts group, you must choose to Delete Existing Contacts during the Google Sync install when prompted. If you choose to keep existing contacts, it will sync the contents of the “All Contacts” group instead. If there are no contacts on your phone, the latter will happen — the contents of your All Contacts group will be synced.
You’ve set up Google Sync! Synchronization will begin automatically if you have Push enabled on your device. You can also open the Mail, Calendar or Contacts app and wait a few seconds to start a sync.
Next choose which calendars to sync, if you’d like to enable multiple calendars.