Google used to tell you how to do this on their website. I cant find it now, so maybe they are no longer supporting this. However, at the time of writing this, it still works for syncing contacts at least.
On the phone, goto Organizer->Synchronization->New Account
Give it a name, such as “Google Sync”
|Username||Your google account username|
|Password||Your google account password|
|Connection||Your Connection Profile|
Then under App. settings, select Contacts and then for Database name use
That should be it. Now you can set it to sync on a schedule. Don’t forget to back up your contacts on both your Google account and on your phone first just in case it all goes wrong.