Jailbreaking the Iphone 3G:

Requirements:

redsn0w_win_0.9.6b5 or later

iPhone1,2_4.2.1_8C148_Restore.ipsw

Installation:

1. Right click on redsnow, and go to  Properties > Compability > Select windows 95

2. Right click on redsnow and run redsnow as admnistrator 

3. Browse and load iPhone1,2_4.2.1_8C148_Restore.ipsw firmware

4. Select "Install Cydia / Select Install IPad baseband (to unlock) / Select Enable battery percentage"

5. Click next and follow the onscreen instructions to enter the DFU mode of the iphone.

6. After the reboot, start cydia (Update if required), add http://cydia.hackulo.us to sources. 

7. Install Ultrasn0w to unlock the Iphone

Restoring the Iphone 3G after modifying the baseband (note: 1015 error if trying to restore using Itunes)

Requirements:

redsn0w_win_0.9.6b5 or later

iPhone1,2_4.2.1_8C148_Restore.ipsw

iPhone-3G-Custom-Firmware-4.2.1-8C148.ipsw

Itunes

Installation:

1. Right click on redsnow, and go to   Properties > Compability > Select windows 95

2. Right click on redsnow and run redsnow as admnistrator 

3. Browse and load iPhone1,2_4.2.1_8C148_Restore.ipsw firmware

4. Select "Just enter pwned DFU mode right now"

5. Click next and follow the onscreen instructions to enter the DFU mode of the iphone

6. Start Itunes and go to the restore screen.

7. Shift + Restore to browse and select iPhone-3G-Custom-Firmware-4.2.1-8C148.ipsw firmware

8. Follow the instructions above to jailbreak/unlock again.