From ModMyi - Wiki

Step 1

First, make sure you are running version 1.0.2 of the iPhone firmware. You can do this by:

Turning off iTunes, and the iTunes helper (Open Applications>Utilities>Activity Monitor, look for iTuneshelper and force quit it).
Connect the iPhone to the computer.
Hold down the Sleep and Home button on the iPhone (top right button and bottom center button) for about 25 seconds until a yellow triangle comes up and it says to connect to iTunes. It will appear to be off for a few seconds, and restart for a few - keep holding til you get that yellow triangle.
Open iTunes and it will say it found an iPhone in Restore mode. Say you want to Restore it, and click Restore on the page it brings you to.
Wait abour 10 minutes to get the new version of the firmware installed.

Step 2

Download the 1.0.2 restore file from Apple. around 91 MB. If you are not using Safari, you will need to rename this file extension to .zip from .ipsw, and right click and uncompress. If you are in Safari, it will automatically do this for you.

Download and run iNdependence 1.1.2a. Make sure you don't use a legacy version of iNdependance still lingering on your computer from previous activations. Drag iNdependance to your hard drive as running it inside the disk image will cause the application to not act properly. Also, make sure iTunes is located in its default install location at /Applications/iTunes.

Step 3

Click Activate iPhone.

iNdependence will ask you where you downloaded the restore file. Navigate to the folder and hit OK. Do NOT choose the file called Firmware, but rather the one containing that folder.

iNdependence will then display Waiting for jail break and your iPhone will go through a few screens, including saying Please connect to iTunes. Do not connect to iTunes, just let it run. This will take around 2-3 minutes.

Then it will say Waiting for return to jail.

You will get a Activation succeeded! message.

Congratulations, your iPhone is now activated.