For the most up to date guide please follow Google's guide HERE
You can also follow the instructions below:
Requirements and Backing Up Data
- Perform a sync with iTunes to ensure that Contacts and Calendars from your iOS device are backed up to your computer. Learn more.
- Check your Safari cookies settings to see if it is set to "From Visited" or "Always". If it's set to "Never", you're more likely to be prompted to solve a CAPTCHA.
- For Google Apps users, your administrator needs to have Google Sync enabled before you can use Google Sync.
Note: After you set up Google Sync, your administrator may also have additional device management capabilities (such as remote wipe or password requirements). Contact your administrator for more details.
- On your device's home screen, open the Settings application.
- Tap Mail, Contacts, Calendars.
- Tap Add Account.
- Tap Exchange. iOS 4.0+ allows multiple Exchange accounts. However, if you're on a device that doesn't let you add a second account, you could also use IMAP to sync Gmail, CalDAV to sync Google Calendar, and CardDAV to sync contacts.
- Enter your full Google Apps email address in the Email field and password in the Password field and tap Next.
- In the Server field, enter m.google.com as your server address. Note: To access m.google.com, set the language toEnglish (US).
- Leave the Domain field blank.
- In the Username field, enter your full Google Apps email address.
- Your password should already be in the Password field from step 5. If it's not, enter your Google Apps password in the Password field. Tap Next.
- Be sure that you have a strong password for your Google Apps account.
- If you see an error message saying "Invalid Password", you may need to unlock your account by solving a short CAPTCHA. Go to https://www.google.com/accounts/UnlockCaptcha.
- If you are a 2-step verification user, please use an Application-specific password instead of your regular password in this field.
- Select the Google services you want to sync, such as Mail, Contacts, and Calendars and tap Save. To receive and respond to meeting requests on your device, you need to turn on both Mail and Calendar and New events in your Google Calendar settings. See below for more information.
Congrats! 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. Note that if you've recently signed up for Google Apps for Work, Education, or Government, it can take a few hours (up to 24) before you're able to sync with your mobile device.