There are a few easy steps:
1. Download the file at 9 to 5 Mac
2. Open the file with iTunes
3. Connect your iPhone and option click the restore button in iTunes and select the open file
4. Go to settings/general/network/internet tethering on your iPhone and turn it on
This will allow you to connect by USB or by Bluetooth; I have tested both and it did not seem to make a difference in speed. What have you found to be faster when tethering? Just remember that if you tether too much AT&T may catch you and can suspend your account or charge you hefty fees. Good luck and happy tethering!